Optical layered body, polarizer, method for producing polarizer, image display device, method for producing image display device, and method for improving visibility of image display device

ABSTRACT

The present invention provides a method for improving visibility of an image display device which is capable of providing an image display device excellent in anti-reflection properties and bright-field contrast even using an optical layered body including a light-transmitting substrate having in-plane birefringence, such as a polyester film. The method of the present invention is a method for improving visibility of an image display device that has an optical layered body including a light-transmitting substrate having in-plane birefringence and an optical functional layer disposed on one surface of the substrate. The method includes the step of disposing the optical layered body such that the slow axis showing a greater refractive index of the light-transmitting substrate is in parallel with the vertical direction of a display screen of the image display device.

TECHNICAL FIELD

The present invention relates to an optical layered body, a polarizer, amethod for producing a polarizer, an image display device, a method forproducing an image display device, and a method for improving visibilityof an image display device.

BACKGROUND ART

Liquid crystal display devices have various advantages such as low powerconsumption, light weight, and slim profile. Owing to these advantages,they have taken the place of conventional CRT displays and have rapidlyspread in these days.

Such a liquid crystal display device is provided with a polarizingelement on the image display screen of its liquid crystal cell, and thepolarizing element usually requires hardness so as to prevent damageswhen handled. In general, such hardness is imparted to an image displayscreen by providing, as a polarizer-protecting film, a hard coat filmthat includes a light-transmitting substrate and a hard coat layerdisposed on the substrate.

Conventionally used light-transmitting substrates for such a hard coatfilm are cellulose ester films such as triacetyl cellulose films. Thisis based on the following advantages of the cellulose esters; that is,the cellulose esters have excellent transparency and optical isotropyand have little in-plane phase difference (have a low retardationvalue), and thus they hardly change the vibration direction of incidentlinearly polarized light and they have less influence on the displayquality of liquid crystal display devices. In addition, the celluloseesters have appropriate water permeability. Thus, when a polarizercomprising an optical layered body is produced, the water remained inthe polarizer can be dried through the optical layered body.

Such a cellulose ester film also has insufficient moisture and heatresistance. Such insufficient characteristics thus deteriorate theproperties of the polarizer such as polarizing function and color phasewhen the hard coat film is used as a polarizer-protecting film in ahigh-temperature and high-humidity environment. Further, a celluloseester is a disadvantageous material in terms of cost.

Such disadvantages of the cellulose ester films provide a demand forusing, as a light-transmitting substrate of an optical layered body, aversatile film that has excellent transparency, heat resistance, andmechanical strength and can be more easily and more inexpensivelyavailable in the market than cellulose ester films or can be produced bya simpler method. For example, a polyester film such as polyethyleneterephthalate is experimentally used as a substitution for a celluloseester film (e.g. see Patent Literature 1 to 3).

However, a polyester film has a high-polarizability aromatic ring in itsmolecular chain and thus has very high intrinsic birefringence.Accordingly, the polyester film has a characteristic of easilyexhibiting birefringence accompanying orientation of the molecular chainas a result of stretching treatment for imparting excellenttransparency, heat resistance, and mechanical strength. An opticallayered body that comprises a light-transmitting substrate havingin-plane birefringence such as the above polyester film, when disposedon the surface of an image display device, may extremely decrease theanti-reflection properties on the surface of the optical layered bodyand may decrease the bright-field contrast.

CITATION LIST Patent Literature

Patent Literature 1: JP 2004-205773 A

Patent Literature 2: JP 2009-157343 A

Patent Literature 3: JP 2010-244059 A

SUMMARY OF INVENTION Technical Problem

The present invention is devised in consideration of the abovesituation, and aims to provide an optical layered body and a polarizereach of which can provide an image display device having excellentanti-reflection properties and bright-field contrast and being capableof preventing a rainbow interference pattern even though alight-transmitting substrate having in-plane birefringence such as apolyester film is used therein, a method for producing the polarizer, animage display device comprising the optical layered body or thepolarizer, a method for producing the image display device, and a methodfor improving visibility of an image display device.

The phrase “visibility is improved” herein means that at least theanti-reflection properties and the bright-field contrast are excellent.Further, the phrase “visibility is well improved” herein means that arainbow interference pattern is additionally prevented.

The phrase “rainbow interference pattern” means unevenness withdifferent colors appearing on a display screen of an image displaydevice when an optical layered body having a conventional polyester filmas a light-transmitting substrate is disposed on the surface of theimage display device and when a person wearing polarized sunglasses seethe display screen. This phenomenon deteriorates display quality.

Solution to Problem

The present invention relates to an optical layered body configured tobe disposed on a surface of an image display device, the optical layeredbody comprising: a light-transmitting substrate having in-planebirefringence; and an optical functional layer disposed on one surfaceof the light-transmitting substrate, the light-transmitting substratehaving a slow axis that is along the direction showing a greaterrefractive index, and the optical layered body being configured to bedisposed on a display screen of the image display device such that theslow axis is in parallel with the vertical direction of the displayscreen.

Preferably, the light-transmitting substrate has a fast axis that isorthogonal to the slow axis, and the difference between refractiveindexes (nx−ny) is 0.05 or greater, where nx represents a refractiveindex in the slow axis direction and ny represents a refractive index inthe fast axis direction.

The light-transmitting substrate in the optical layered body of thepresent invention preferably has a retardation of 3000 nm or greater.

The light-transmitting substrate is preferably a substrate formed from apolyester, and the polyester is preferably polyethylene terephthalate orpolyethylene naphthalate.

Preferably, the optical layered body of the present invention furthercomprises a primer layer disposed between the light-transmittingsubstrate and the optical functional layer, wherein the primer layer is3 to 30 nm in thickness provided that: the primer layer has a refractiveindex (np) that is greater than the refractive index (nx) in the slowaxis direction of the light-transmitting substrate and that is greaterthan the refractive index (nf) of the optical functional layer (np>nxand np>nf), or the primer layer has a refractive index (np) that issmaller than the refractive index (ny) in the fast axis direction of thelight-transmitting substrate and that is smaller than the refractiveindex (nf) of the optical functional layer (np<ny and np<nf).

Preferably, the optical layered body of the present invention furthercomprises a primer layer disposed between the light-transmittingsubstrate and the optical functional layer, wherein the primer layer is65 to 125 nm in thickness provided that: the primer layer has arefractive index (np) that is greater than the refractive index (nx) inthe slow axis direction of the light-transmitting substrate but that issmaller than the refractive index (nf) of the optical functional layer(nx<np<nf), or the primer layer has a refractive index (np) that issmaller than the refractive index (ny) in the fast axis direction of thelight-transmitting substrate but that is greater than the refractiveindex (nf) of the optical functional layer (nf<np<ny).

Preferably, the optical layered body of the present invention furthercomprises a primer layer disposed between the light-transmittingsubstrate and the optical functional layer, wherein the primer layer hasa refractive index (np) that falls between the refractive index (ny) inthe fast axis direction of the light-transmitting substrate and therefractive index (nx) in the slow axis direction of thelight-transmitting substrate (ny<np<nx).

The present invention also relates to a polarizer that is configured tobe disposed on a surface of an image display device, the polarizercomprising: a polarizing element; and an optical layered body disposedon the polarizing element, the optical layered body including: alight-transmitting substrate having in-plane birefringence; and anoptical functional layer disposed on one surface of thelight-transmitting substrate, the light-transmitting substrate having aslow axis with a greater refractive index, the polarizing element havingan absorption axis, the optical layered body and the polarizing elementbeing disposed such that the slow axis of the light-transmittingsubstrate and the absorption axis of the polarizing element areorthogonal to each other, and the polarizer being configured to bedisposed on a display screen of the image display device such that theslow axis of the light-transmitting substrate is in parallel with thevertical direction of the display screen.

Preferably, the light-transmitting substrate having in-planebirefringence in the polarizer of the present invention further has afast axis that is orthogonal to the slow axis, and the differencebetween the refractive indexes (nx−ny) is 0.05 or greater, where nxrepresents a refractive index in the slow axis direction and nyrepresents a refractive index in the fast axis direction.

The light-transmitting substrate having in-plane birefringencepreferably has a retardation of 3000 nm or greater.

Preferably, the polarizer of the present invention further comprises aprimer layer disposed between the light-transmitting substrate and theoptical functional layer, wherein the primer layer is 3 to 30 nm inthickness provided that: the primer layer has a refractive index (np)that is greater than the refractive index (nx) in the slow axisdirection of the light-transmitting substrate and that is greater thanthe refractive index (nf) of the optical functional layer (np>nx andnp>nf), or the primer layer has a refractive index (np) that is smallerthan the refractive index (ny) in the fast axis direction of thelight-transmitting substrate and that is smaller than the refractiveindex (nf) of the optical functional layer (np<ny and np<nf).

Preferably, the polarizer of the present invention further comprises aprimer layer disposed between the light-transmitting substrate and theoptical functional layer, wherein the primer layer is 65 to 125 nm inthickness provided that: the primer layer has a refractive index (np)that is greater than the refractive index (nx) in the slow axisdirection of the light-transmitting substrate but that is smaller thanthe refractive index (nf) of the optical functional layer (nx<np<nf), orthe primer layer has a refractive index (np) that is smaller than therefractive index (ny) in the fast axis direction of thelight-transmitting substrate but that is greater than the refractiveindex (nf) of the optical functional layer (nf<np<ny).

Preferably, the polarizer of the present invention further comprises aprimer layer disposed between the light-transmitting substrate and theoptical functional layer, wherein the primer layer has a refractiveindex (np) that falls between the refractive index (ny) in the fast axisdirection of the light-transmitting substrate and the refractive index(nx) in the slow axis direction of the light-transmitting substrate(ny<np<nx).

The present invention also relates to an image display device comprisingthe optical layered body of the present invention or the polarizer ofthe present invention.

The image display device of the present invention is preferably aVA-mode or IPS-mode liquid crystal display device comprising awhite-light-emitting diode as a backlight light source.

The present invention also relates to a method for producing an imagedisplay device, the image display device including an optical layeredbody that has a light-transmitting substrate having in-planebirefringence and an optical functional layer disposed on one surface ofthe light-transmitting substrate, the light-transmitting substratehaving a slow axis that extends along the direction showing a greaterrefractive index, the method comprising disposing the optical layeredbody such that the slow axis of the light-transmitting substrate is inparallel with the vertical direction of a display screen of the imagedisplay device.

The present invention also relates to a method for improving visibilityof an image display device, the image display device including anoptical layered body that has a light-transmitting substrate havingin-plane birefringence and an optical functional layer disposed on onesurface of the light-transmitting substrate, the light-transmittingsubstrate having a slow axis that extends along the direction showing agreater refractive index, the method comprising disposing the opticallayered body such that the slow axis of the light-transmitting substrateis in parallel with the vertical direction of a display screen of theimage display device.

The present invention will be described in detail below.

In the present invention, curable resin precursors such as monomers,oligomers, and pre-polymers are also referred to as “resin” unlessotherwise mentioned.

As a result of diligent studies, the present inventors have found thatan image display device with excellent anti-reflection properties andbright-field contrast can be provided by disposing an optical layeredbody or a polarizer that comprises a light-transmitting substrate havingin-plane birefringence on the image display device such that the slowaxis, which extends along the direction with a greater refractive index,of the light-transmitting substrate extends in a specific directionrelative to the absorption axis of the polarizing element or a displayscreen of the image display device. Thereby, the present inventors havecompleted the present invention. The cellulose ester film, such as atriacetyl cellulose film, conventionally used as an optical layered bodyas mentioned above is excellent in optical isotropy and hardly hasin-plane phase difference. Thus, an optical layered body or a polarizercomprising the cellulose ester film as a light-transmitting substraterequires no consideration in the direction of disposing thelight-transmitting substrate. In other words, the aforementioneddisadvantages about the anti-reflection properties and the bright-fieldcontrast arise from the use of a light-transmitting substrate havingin-plane birefringence as the light-transmitting substrate of an opticallayered body.

A liquid crystal display device looked over a polarized sunglassesdisadvantageously exhibits deteriorated visibility depending on theangle between the absorption axis of the polarizer of the liquid crystaldisplay device and the absorption axis of the polarized sunglasses. Oneknown method for improving this visibility is to dispose alight-transmitting substrate having in-plane birefringence such as a λ/4phase-difference film at a position closer to the observer than theobserver-side polarizer in the liquid crystal display device. This is amethod of controlling the intensity of transmitted light that ismeasured under the crossed-nicols state and that is represented by thefollowing formula:I=I ₀·sin²(2θ)·sin²(π·Re/λ)wherein θ represents the angle between the absorption axis of thepolarizer and the slow axis of the light-transmitting substrate havingin-plane birefringence; I represents the intensity of light passedthrough the crossed-nicols state; I₀ represents the intensity of lightentering the light-transmitting substrate having in-plane birefringence;λ represents the wavelength of light; and Re represents the retardation.

The value of sin²(2θ) depends on the value of θ and is 0 to 1. In themethod for improving visibility where the display device is looked overpolarized sunglasses, the value θ is set to 45° in many cases so as toachieve sin²(2θ)=1, thereby providing a greater intensity of the passinglight.

However, the optical layered body of the present invention is devised onthe basis of a technical idea that is completely different from theaforementioned technique using polarized sunglasses based on the aboveformula.

The optical layered body of the present invention has alight-transmitting substrate having in-plane birefringence and anoptical functional layer disposed on one surface of thelight-transmitting substrate. The optical layered body is configured tobe disposed on the surface of an image display device such that the slowaxis extending along the direction showing a greater refractive index ofthe light-transmitting substrate is in parallel with the verticaldirection of a display screen of the image display device.

An image display device is usually placed in a room. Thus, prevention ofreflection of light that is reflected on a wall surface or floor surfaceon the display screen (the surface of the optical layered body) of theimage display device enables to give excellent anti-reflectionproperties.

The present inventors have focused on the fact that most part of thelight reflected on a wall surface or floor surface and entering thedisplay screen of the image display device vibrates in the horizontaldirection of the display screen. Then, they have designed that theoptical layered body of the present invention is disposed such that theslow axis extending along the direction showing a greater refractiveindex of the light-transmitting substrate is in parallel with thevertical direction of the display screen of the image display device. Inother words, the optical layered body of the present invention islimited to be disposed on the surface of an image display device, and animage display device having the optical layered body of the presentinvention disposed thereon satisfies that the slow axis extending alongthe direction showing a greater refractive index of thelight-transmitting substrate is orthogonal to the vibration direction ofthe light reflected on a wall surface or floor surface. As mentionedhere, the image display device having an optical layered body disposedthereon such that the slow axis extending along the direction showing agreater refractive index of the light-transmitting substrate is in aspecific direction is excellent in anti-reflection properties andbright-field contrast.

This is because the image display device having the optical layered bodyof the present invention disposed thereon in the aforementioned specificstate satisfies that the fast axis, which extends along the directionshowing a smaller refractive index, of the light-transmitting substrateis in parallel with the light (S-polarized light) that vibrates in thehorizontal direction where a greater part of the light enters thedisplay screen, and thus can reduce reflection of natural light on theoutermost surface.

The reason of this is as follows. The reflectance R on the surface of asubstrate having a refractive index N is represented by the formula:R=(N−1)²/(N+1)².

For a substrate having refractive index anisotropy such as thelight-transmitting substrate in the optical layered body of the presentinvention, the optical layered body having the aforementioned structurein the image display device increases the ratio of the refractive indexof the fast axis with a smaller refractive index to be applied to therefractive index N.

Because of the above reason, the reflectance in the case where theoptical layered body comprising a light-transmitting substrate havingin-plane phase difference is disposed on an image display device withoutthe consideration of the direction of disposing the substrate is greaterthan the reflectance in the case where such an optical layered body isdisposed such that the slow axis direction with a greater refractiveindex of the light-transmitting substrate is in a specific direction asin the present invention. The phrase “excellent in anti-reflectionproperties” in the present invention means the aforementioned state.

For the optical layered body of the present invention that is disposedon an image display device so as to have the aforementionedconfiguration, the reflectance thereof is preferably similar to thatachieved by a film as a substrate that is excellent in optical isotropyand hardly has an in-plane phase difference (e.g. cellulose ester filmsuch as triacetyl cellulose film). For example, a film formed fromtriacetyl cellulose has a reflectance of about 4.39%.

The contrast of an image display device consists of the dark-fieldcontrast and the bright-field contrast. The dark-field contrast iscalculated based on the formula (luminance in white screen)/(luminancein black screen), whereas the bright-field contrast is calculated basedon the formula {(luminance in white screen+natural lightreflection)/(luminance in black screen+natural light reflection)}. Ineither contrast, a greater influence of the denominator leads to a lowercontrast. In other words, reduction in the natural light reflection onthe outermost surface leads to an increase in the bright-field contrast.

The phrase “an optical layered body is disposed on an image, displaydevice such that the slow axis extending along the direction showing agreater refractive index of the light-transmitting substrate is inparallel with the vertical direction of a display screen of the imagedisplay device” means that the optical layered body is disposed on theimage display device such that the slow axis forms an angle of 0°±40°with the vertical direction of the display screen.

In the optical layered body of the present invention, the angle betweenthe slow axis of the light-transmitting substrate and the verticaldirection of the display screen is preferably 0°±30°, more preferably0°±10°, and still more preferably 0°±5°. An angle of 0°±40° between theslow axis of the light-transmitting substrate and the vertical directionof the display screen in disposing the optical layered body of thepresent invention enables to improve the bright-field contrast owing tothe optical layered body of the present invention.

In order to improve the bright-field contrast owing to the opticallayered body of the present invention, the angle between the slow axisof the light-transmitting substrate and the vertical direction of thedisplay screen is most preferably 0°. Thus, the angle between the slowaxis of the light-transmitting substrate and the vertical direction ofthe display screen is preferably 0°±30°, and more preferably 0°±10°,rather than 0°±40°. Further, the angle between the slow axis of thelight-transmitting substrate and the vertical direction of the displayscreen is still more preferably 0°±5° because such an angle enables toimprove the bright-field contrast as much as the angle of 0°.

With respect to the angle between the above two axes seen from theobserver side herein, the angle formed in the clockwise directionrelative to the substrate angle is defined as plus (+), whereas theangle formed in the counterclockwise direction relative to the substrateangle is defined as minus (−). Angles with no specific symbol are eachan angle formed in the clockwise direction relative to the substrateangle (that is, the angle is a plus angle).

The light-transmitting substrate having in-plane birefringence is notparticularly limited, and examples thereof include substrates formedfrom polycarbonates, acrylics, and polyesters. Preferable are polyestersubstrates that are advantageous in terms of cost and mechanicalstrength. The following description will be made using a polyestersubstrate as the light-transmitting substrate having in-planebirefringence.

The polyester substrate preferably has a retardation of 3000 nm orhigher so as to prevent a rainbow interference pattern and provide verygood visibility. A retardation of lower than 3000 nm may cause sightingof a rainbow interference pattern like a rainbow-colored striped patternand reduction in display quality when the optical layered body of thepresent invention is used for a liquid crystal display device (LCD). Theupper limit of the retardation of the polyester substrate is notparticularly limited, and is preferably about 30,000 nm. A substratewith a retardation exceeding 30,000 nm may be considerably thick andthus is not preferred.

The retardation of the polyester substrate is preferably 5,000 to 25,000nm for film thinning. The retardation is more preferably 7,000 to 20,000nm. A retardation within this range enables to prevent a rainbowinterference pattern more favorably even though the optical layered bodyof the present invention is disposed on an image display device suchthat the slow axis of the polyester substrate forms an angle of 0°±30°to 0°±40° with the vertical direction of the display screen, in otherwords, the slow axis of the polyester substrate and the verticaldirection of the display screen form an angle that slightly shifts fromthe perfectly parallel state. Even though the slow axis of the polyestersubstrate forms an angle of ±30° to 40° from the perfectly parallelstate with the vertical direction of the display screen, the opticallayered body of the present invention with a retardation of 3000 nm orhigher enables to prevent a rainbow interference pattern, and thus hasno disadvantages in practical use. Nevertheless, the angle between theslow axis of the light-transmitting substrate and the vertical directionof the display screen is most preferably 0° as mentioned above.Therefore, the angle between the slow axis of the light-transmittingsubstrate and the vertical direction of the display screen is morepreferably 0°±10°, and still more preferably 0°±5°.

The retardation is represented by the following formula:Retardation (Re)=(nx−ny)×dwherein nx represents the refractive index in the direction with thegreatest refractive index in the plane of the polyester substrate (slowaxis direction); ny represents the refractive index in the directionorthogonal to the slow axis direction (fast axis direction); and drepresents the thickness of the polyester substrate.

The retardation can be measured using KOBRA-WR (Oji ScientificInstruments) (measurement angle: 0°; measurement wavelength: 589.3 nm),for example.

Two polarizers are used to determine the direction of orientation axis(the direction of principal axis) of the polyester substrate, and thenthe refractive indexes (nx and ny) of the two axes orthogonal to thedirection of polarization axis are measured using an Abbe refractometer(NAR-4T, ATAGO CO., LTD,). The axis showing a greater refractive indexis defined as the slow axis. The thickness d (nm) of the polyestersubstrate is measured using an electric micrometer (ANRITSU CORP.) andthe unit is converted into nanometer. The retardation may be calculatedby multiplying the difference between the refractive indexes (nx−ny) bythe thickness d (nm) of the film.

The refractive index can be measured using an Abbe refractometer or anellipsometer, or can be measured using a spectrophotometer (UV-3100PC,Shimadzu Corp.) as follows; that is, the average reflectance (R) of theoptical functional layer at a wavelength of 380 to 780 nm of the opticallayered body of the present invention is first measured, and then therefractive index (n) is calculated from the obtained average reflectance(R) based on the below-mentioned formula.

The average reflectance (R) of the optical functional layer is measuredas follows. A material composition is applied onto one surface of each aPET film with a thickness of 50 μm without primer treatment and eachmaterial composition applied is formed into a curable film with athickness of 1 to 3 μm; the other surface (back side) of the PET with nocoating is covered by a black plastic tape (e.g. Yamato Vinyl Tape No200-38-21, 38 mm width) that is wider than the measurement-spot area forprevention of back reflection; and the average reflectance of eachcurable film is measured. The refractive index of the polyestersubstrate is also measured after a black plastic tape is attached ontothe side opposite to the measurement side.R(%)=(1−n)²/(1+n)²

Examples of a method for measuring the refractive index of the opticalfunctional layer after formation of an optical layered body include amethod of shaving off the cured film of each layer using, for example, acutter to prepare a powdery sample and then performing the Becke methodon the sample in conformity with the B method in JIS K7142 (2008) (forpowdery or granular transparent materials). The Becke method is a methodincluding: placing a powdery sample on, for example, a glass slide;dripping a Cargille reagent with a known refractive index onto thesample to immerse the sample in the reagent; microscopically observingthe state of immersion; and determining a reagent that provides nobright line (Becke line), which occurs along the sample outline when thesample and the reagent have different refractive indexes, in the visualobservation. The refractive index of such a reagent is defined as therefractive index of the sample.

Since the polyester substrate has different refractive indexes (nx andny) in different directions, the refractive indexes can also be measurednot by the Becke method but by the following method. That is, a blackplastic tape is attached to a treated surface of the optical functionallayer; the 5-degree reflectance of the light-transmitting substrate withthe slow axis set in parallel and that with the fast axis set inparallel are measured using S-polarized light with a spectrophotometer(automatic absolute reflectance measurement unit V7100-series, VAR-7010,JASCO Corp.) (polarized light measurement); and the refractive indexes(nx and ny) in the slow axis and the fast axis can be calculated on thebasis of the aforementioned formula.

In the case where the polyester substrate is a PET substrate formed frompolyethylene terephthalate (PET) to be mentioned later in the presentinvention, the (nx−ny) value (hereinafter also referred to as Δn) ispreferably 0.05 or higher. The substrate with a Δn value of lower than0.05 may have too high a refractive index in the fast axis, and thus mayfail to improve the bright-field contrast of an image display device.Further, the substrate may disadvantageously require a greater thicknessso as to achieve the aforementioned retardation. On the other hand, theΔn is preferably 0.25 or lower. The PET substrate with a Δn exceeding0.25 may need excessive stretching. This excessive stretching easilycauses the PET substrate to suffer rents, tears, or the like, andthereby markedly deteriorates the practicality of the substrate as anindustrial material.

From the above points of view, the lower limit of the Δn of the PETsubstrate is more preferably 0.07, whereas the upper limit thereof ismore preferably 0.20. The PET substrate with a Δn exceeding 0.20 mayhave poor durability in a moisture and heat resistance test. The upperlimit of the Δn of the PET substrate is more preferably 0.15 forexcellent durability of the substrate in the moisture and heatresistance test.

The nx value of the PET substrate is preferably 1.66 to 1.78. The lowerlimit thereof is more preferably 1.68, whereas the upper limit thereofis more preferably 1.73. The ny value of the PET substrate is preferably1.55 to 1.65. The lower limit thereof is more preferably 1.57, whereasthe upper limit thereof is more preferably 1.62.

The nx and ny values within the above range and satisfying the aboverelationship about the Δn enable to favorably improve theanti-reflection properties and the bright-field contrast.

In the case where the polyester substrate is a PEN substrate formed frompolyethylene naphthalate (PEN) to be mentioned later, the lower limit ofthe Δn is preferably 0.05, whereas the upper limit thereof is preferably0.30. The substrate with a Δn of lower than 0.05 is not preferredbecause it may require a greater thickness so as to achieve theaforementioned retardation. In contrast, the PEN substrate with a Δnexceeding 0.30 may easily suffer rents, tears, or the like, and maythereby have markedly deteriorated practicality as an industrialmaterial. The lower limit of the Δn of the PEN substrate is morepreferably 0.07, whereas the upper limit thereof is more preferably0.27. A Δn of lower than 0.07 may have difficulty in achieving theaforementioned effects of sufficiently preventing a rainbow interferencepattern and color-tone change. The PEN substrate with a Δn exceeding0.27 may have poor durability in the moisture and heat resistance test.The upper limit of the Δn of the PEN substrate is still more preferably0.25 for excellent durability in the moisture and heat resistance test.

The nx value of the PEN substrate is preferably 1.70 to 1.90. The lowerlimit thereof is more preferably 1.72, whereas the upper limit thereofis more preferably 1.88. The ny value of the PEN substrate is preferably1.55 to 1.75. The lower limit thereof is more preferably 1.57, whereasthe upper limit thereof is more preferably 1.73.

Any material satisfying the above retardation may be used for thepolyester substrate. Examples thereof include linear saturatedpolyesters synthesized from any of aromatic dibasic acids andester-forming derivatives thereof and any of diols and ester-formingderivatives thereof. Specific examples of the polyesters includepolyethylene terephthalate, polyethylene isophthalate, polybutyleneterephthalate, poly(1,4-cyclohexylene dimethylene terephthalate), andpolyethylene naphthalate (e.g. polyethylene-2,6-naphthalate,polyethylene-1,4-naphthalate, polyethylene-1,5-naphthalate,polyethylene-2,7-naphthalate, and polyethylene-2,3-naphthalate). Thepolyester used for the polyester substrate may be a copolymer of thesepolyesters, or may be one prepared by blending a polyester as a maincomponent (for example, 80 mol % or more) and one or more other resinseach in a small amount (for example, 20 mol % or lower). The polyesteris particularly preferably polyethylene terephthalate or polyethylenenaphthalate because of their good balance of properties such asmechanical physical properties and optical characteristics. Thepolyester is particularly preferably polyethylene terephthalate (PET).This is because polyethylene terephthalate is very versatile and isreadily available. The present invention enables to provide an opticallayered body which provides a liquid crystal display device with highdisplay quality even with a very versatile film such as PET. Further,PET is excellent in transparency and in heat or mechanicalcharacteristics, the retardation of which may be controlled bystretching. In addition, PET has high intrinsic birefringence. Thus,even a thin substrate relatively easily has a high retardation.

The polyester substrate may be obtained by any method that provides theabove retardation. Examples thereof include a method in which a materialpolyester such as PET is molten; the molten polyester isextrusion-molded into a sheet to prepare an unstretched polyester; theunstretched polyester is stretched in the transverse direction at atemperature not lower than the glass transition temperature using, forexample, a tenter; and then the stretched film is subjected to heattreatment.

The transverse direction stretching temperature is preferably 80° C. to130° C., and more preferably 90° C. to 120° C. The transverse directionstretching ratio is preferably 2.5 to 6.0 times, and more preferably 3.0to 5.5 times. A transverse direction stretching ratio exceeding 6.0times may easily decrease the transparency of the polyester substrate. Astretching ratio of lower than 2.5 times may cause a lower stretchingtension, thereby providing low birefringence of the polyester substrateand failing to provide a retardation of 3000 nm or higher.

In the present invention, the transverse direction stretching of theunstretched polyester under the aforementioned conditions may befollowed by stretching in the flow direction (hereinafter, also referredto as machine direction stretching) using a biaxial stretching testerrelative to the transverse direction stretching. In this case, thestretching ratio of the machine direction stretching is preferably twiceor lower. A stretching ratio of higher than twice in the machinedirection stretching may fail to set the Δn within the above preferablerange.

The temperature at the heat treatment is preferably 100° C. to 250° C.,and more preferably 180° C. to 245° C.

A conventional polyester substrate is obtained by stretching anunstretched polyester substrate in the longitudinal direction and thenstretching it in the width direction at a similar ratio for thelongitudinal direction stretching. However, the polyester substrateobtained by such a stretching method easily suffers a bowing phenomenon.In contrast, the polyester substrate having the above retardation valueof the present invention can be obtained by stretching a roll-shapedupstretched optically transparent film only in the width direction or byslightly stretching the film in the machine direction and thenstretching the film in the width direction. The polyester substrate thusobtained enables to prevent the bowing phenomenon and its slow axisextends along the width direction.

As is mentioned later, the optical layered body of the present inventionmay be disposed on a polarizing element to form a polarizer. Here, theroll-shaped polarizing element has its absorption axis along thelongitudinal direction. Thus, attachment of the light-transmittingsubstrate and the polarizing element by roll-to-roll processing enablesto form a polarizer where the absorption axis of the polarizing elementand the slow axis of the light-transmitting substrate are at rightangle. Such an angle between the slow axis of the light-transmittingsubstrate and the absorption axis of the polarizing element will bementioned later.

Appropriate adjustment of the stretching ratio, the stretchingtemperature, the thickness of a polyester substrate to be obtained, andthe like enables to adjust the retardation of the polyester substrateproduced by the above method to 3000 nm or higher. Specifically, ahigher stretching ratio, a lower stretching temperature, and a greaterthickness make it easy to provide a higher retardation. In contrast, alower stretching ratio, a higher stretching temperature, and a smallerthickness make it easy to provide a lower retardation.

The polyester substrate is preferably 40 to 500 μm in thickness. Thepolyester substrate with a thickness of less than 40 μm may fail to havea retardation of 3000 nm or higher and may clearly have anisotropy ofthe mechanical characteristics. Thereby, the substrate may easily sufferrents, tears, or the like and may markedly deteriorate in thepracticality as an industrial material. In contrast, the polyestersubstrate with a thickness exceeding 500 μm may disadvantageously bevery rigid and may deteriorate in flexibility that is characteristic ofpolymeric films, thereby resulting in deterioration in the practicalityas an industrial material. The lower limit of the thickness of thepolyester substrate is more preferably 50 μm, whereas the upper limitthereof is more preferably 400 μm, and still more preferably 300 μm.

The polyester substrate preferably has a transmittance within thevisible light region of 80% or higher, and more preferably 84% orhigher. The transmittance can be measured in conformity with JIS K7361-1(Plastics—Determination of the total luminous transmittance oftransparent materials).

In the present invention, the polyester substrate may be surface-treatedwithin the scope of the present invention. Examples of the surfacetreatment include saponification, glow discharge, corona discharge,ultraviolet (UV) treatment, and flame treatment.

The optical functional layer is preferably a hard coat layer having ahard coat property. The hard coat layer preferably has a hardness of Hor higher, and more preferably 2H or higher, in conformity with a pencilhardness test (load: 4.9 N) in JIS K5600-5-4 (1999).

The hard coat layer is a layer that guarantees the hard coat property onthe surface of the optical layered body of the present invention. Forexample, the hard coat layer is preferably formed from a composition fora hard coat layer containing an ionizing-radiation-curable resin, whichis a resin curable by ultraviolet rays, and a photo-polymerizationinitiator.

Examples of the ionizing-radiation-curable resin used for the opticallayered body of the present invention include compounds having one ortwo or more unsaturated bonds such as acrylic functionalgroup-containing compounds. Examples of the compounds having oneunsaturated bond include ethyl (meth)acrylate, ethylhexyl(meth)acrylate, styrene, methylstyrene, and N-vinylpyrrolidone. Examplesof the compounds having two or more unsaturated bonds includepolymethylolpropane tri(meth)acrylate, tripropylene glycoldi(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritoltri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, dipentaerythritolpenta(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentylglycol di(meth)acrylate; multi-functional compounds obtained bymodifying any of the above compounds with ethylene oxide (EO); andreaction products between any of the multi-functional compounds and anyof (meth)acrylates (e.g. poly(meth)acrylate esters of polyhydricalcohols). The term “(meth)acrylate” herein includes methacrylate andacrylate.

In addition to the above compounds, those having a relatively lowmolecular weight with an unsaturated double bond (number averagemolecular weight: 300 to 80,000, preferably 400 to 5,000), such aspolyester resin, polyether resin, acrylic resin, epoxy resin, urethaneresin, alkyd resin, spiroacetal resin, polybutadiene resin, andpolythiol polyene resin, may be used as the ionizing-radiation-curableresin. The “resin” herein includes all of dimers, oligomers, andpolymers, excluding monomers.

Examples of preferred compounds in the present invention includecompounds having three or more unsaturated bonds. Such a compoundenables to increase the cross-linking density of a hard coat layer andto provide a good coating hardness.

Specifically preferred in the present invention is appropriatecombination of pentaerythritol triacrylate, pentaerythritoltetraacrylate, polyester multi-functional acrylate oligomers (3 to 15functions), urethane multi-functional acrylate oligomers (3 to 15functions), and the like.

The ionizing-radiation-curable resin may be used in combination with asolvent-drying-type resin. Combination use with a solvent-drying-typeresin enables to effectively prevent defects of the coating. The“solvent-drying-type resin” herein means a resin which contains asolvent that is added upon application of the resin for adjustment ofthe solids content and which turns into a coating only by drying of thesolvent. Examples of such a resin include thermoplastic resin.

The solvent-drying-type resin to be used in combination with theionizing-radiation-curable resin is not particularly limited, and anythermoplastic resin may be used, in general.

The thermoplastic resin is not particularly limited. Examples thereofinclude styrenic resin, (meth)acrylic resin, vinyl acetate resin, vinylether resin, halogen-containing resin, alicyclic olefinic resin,polycarbonate resin, polyester resin, polyamide resin, cellulosederivatives, silicone resin, rubber, and elastomers. The thermoplasticresin is preferably amorphous and soluble in an organic solvent(especially, a common solvent which dissolves various polymers andcurable compounds). Particularly preferred from the viewpoints of filmformability, transparency, and weather resistance are styrenic resin,(meth)acrylic resin, alicyclic olefinic resin, polyester resin, andcellulose derivatives (e.g. cellulose esters).

The composition for a hard coat layer may contain a thermosetting resin.

The thermosetting resin is not particularly limited. Examples thereofinclude phenol resin, urea resin, diallyl phthalate resin, melamineresin, guanamine resin, unsaturated polyester resin, polyurethane resin,epoxy resin, aminoalkyd resin, melamine-urea co-condensed resin, siliconresin, and polysiloxane resin.

The photo-polymerization initiator is not particularly limited and knownones may be used. Specific examples of the photo-polymerizationinitiator include acetophenones, benzophenones, Michler's benzoylbenzoate, α-amyloxim ester, thioxanthones, propiophenones, benzils,benzoins, and acyl phosphine oxides. The initiator is preferably used inadmixture with a photosensitizer. Specific examples thereof includen-butyl amine, triethyl amine, and poly-n-butyl phosphine.

For a resin having a radical polymerizable unsaturated group as theionizing-radiation-curable resin, the photo-polymerization initiator ispreferably one of acetophenones, benzophenones, thioxanthones, benzoin,and benzoin methyl ether, or a mixture thereof. For a resin havingcation polymerizable functional group as the ionizing-radiation-curableresin, the photo-polymerization initiator is preferably one of aromaticdiazonium salts, aromatic sulfonium salts, aromatic iodonium salts,metallocene compounds, and benzoin sulfonic ester, or a mixture thereof.

For the ionizing-radiation-curable resin having a radical polymerizableunsaturated group, the initiator used in the present invention ispreferably 1-hydroxy-cyclohexyl-phenyl-ketone because this substance iswell compatible with ionizing-radiation-curable resin and less causesyellowing.

The amount of the photo-polymerization initiator in the composition fora hard coat layer is preferably 1 to 10 parts by mass for 100 parts bymass of the ionizing-radiation-curable resin. If the amount thereof isless than 1 part by mass, the hard coat layer in the optical layeredbody of the present invention may fail to have a hardness within theaforementioned range. If the amount thereof is more than 10 parts bymass, ionizing radiation may not reach the depth of the formed film andmay fail to urge internal curing. Thereby, the hard coat layer may failto have a desired pencil hardness of 3H or higher on its surface.

The lower limit of the amount of the photo-polymerization initiator ismore preferably 2 parts by mass, whereas the upper limit thereof is morepreferably 8 parts by mass. The photo-polymerization initiator in anamount within this range enables to prevent hardness distribution in thethickness direction and to easily provide a uniform hardness.

The composition for a hard coat layer may contain a solvent.

The solvent to be used depends on the type and solubility of the resincomponent to be used. Examples thereof include ketones (e.g. acetone,methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, diacetonealcohol), ethers (e.g. dioxane, tetrahydrofuran, propylene glycolmonomethyl ether, propylene glycol monomethyl ether acetate), aliphatichydrocarbons (e.g. hexane), alicyclic hydrocarbons (e.g. cyclohexane),aromatic hydrocarbons (e.g. toluene, xylene), halogenated hydrocarbons(e.g. dichloromethane, dichloroethane), esters (e.g. methyl acetate,ethyl acetate, butyl acetate), water, alcohols (e.g. ethanol,isopropanol, butanol, cyclohexanol), cellosolves (e.g. methylcellosolve, ethyl cellosolve), cellosolve acetates, sulfoxides (e.g.dimethyl sulfoxide), amides (e.g. dimethyl formamide, dimethylacetamide). These solvents may also be used in admixture.

Particularly preferred in the present invention are ketone-typesolvents, especially one of methyl ethyl ketone, methyl isobutyl ketone,and cyclohexanone or a mixture thereof because they are well compatiblewith resin and are easy to apply.

The proportion of the materials (solids content) in the composition fora hard coat layer is not particularly limited, and it is generally 5 to70% by mass, and particularly preferably 25 to 60% by mass.

For the purposes of increasing the hardness of the hard coat layer,suppressing curing shrinkage, preventing blocking, adjusting therefractive index, imparting an anti-glare property, modifying theproperties of the particles and the surface of hard coat layer, and thelike, the composition for a hard coat layer may contain any ofconventionally known organic and inorganic fine particles, dispersingagents, surfactants, antistatic agents, silane-coupling agents,thickening agents, anti-coloring agents, coloring agents (pigments,dyes), anti-foaming agents, leveling agents, flame retarders,ultraviolet absorbers, adhesion promoters, polymerization inhibitors,antioxidants, surface modifiers, and the like additives.

The composition for a hard coat layer may contain a photosensitizer.Specific examples thereof include n-butyl amine, triethyl amine, andpoly-n-butyl phosphine.

The composition for a hard coat layer may be prepared by any methodwhich allows the components to be uniformly mixed. The method may be oneusing any known device such as a paint shaker, a bead mill, a kneader,or a mixer.

The composition for a hard coat layer may be applied onto thelight-transmitting substrate by any method. Examples of the methodinclude known methods such as gravure coating, spin coating, dipping,spraying, die coating, bar coating, roll coating, meniscus coating,flexo printing, screen printing, and bead coating.

The film formed by application of the composition for a hard coat layeronto the light-transmitting substrate is preferably heated and/or driedas appropriate and then cured by, for example, irradiation of activeenergy rays.

The irradiation of active energy rays may be irradiation of ultravioletrays or an electron beam. Specific examples of the source of ultravioletrays include an ultra-high-pressure mercury lamp, a high-pressuremercury lamp, a low-pressure mercury lamp, a carbon arc lamp, a blacklight fluorescent lamp, and a metal halide lamp. The wavelength of theultraviolet rays may be 190 to 380 nm. Specific examples of the sourceof an electron beam include various electron linear accelerators ofCockcroft-Walton type, Van de graaff type, resonant transformer type,insulated core transformer type, linear type, dynamitron type, andhigh-frequency type.

The thickness (after cured) of the hard coat layer is preferably 0.5 to100 μm, and more preferably 0.8 to 20 μm. In order to achieve excellentproperties of preventing curling and cracking, the thickness is mostpreferably 2 to 10 μm. The thickness of the hard coat layer is anaverage value (μm) of arbitrarily selected 10 points in the crosssection observed through an electron microscope (SEM, TEM, STEM). Thethickness of the hard coat layer may be determined by another method;that is, for example, arbitrarily selected 10 points are measured usinga thickness measurement device (Digimatic Indicator IDF-130, MitutoyoCorp.).

Blending an antistatic agent into the composition for a hard coat layerenables to impart an antistatic property to the hard coat layer.

The antistatic agent may be a conventionally known one. Examples thereofinclude cationic antistatic agents such as quaternary ammonium salts,fine particles such as tin-doped indium oxide (ITO) particles, andconductive polymers.

In the case of using the antistatic agent, The amount thereof ispreferably 1 to 30% by mass for the total solids content.

The optical layered body of the present invention preferably further hasa low refractive layer on the hard coat layer.

The low refractive layer is preferably formed from 1) resin containinglow refractive inorganic fine particles such as silica or magnesiumfluoride, 2) fluororesin which is a low refractive index resin, 3)fluororesin containing low refractive inorganic fine particles such assilica or magnesium fluoride, or 4) a low refractive inorganic thin filmsuch as silica or magnesium fluoride. Resins other than the fluororesinsmay be the same as the aforementioned resins.

The silica is preferably hollow silica particles. Such hollow silicaparticles can be produced by the method disclosed in Examples of JP2005-099778 A, for example.

The low refractive layer preferably has a refractive index of 1.47 orlower, and more preferably 1.42 or lower.

The low refractive layer may have any thickness. In general, thethickness may be appropriately selected within about 10 nm to about 1μm.

The fluororesin may be a polymerizable compound at least having afluorine atom in its molecule or a polymer of such a compound. Thepolymerizable compound is not particularly limited, and it preferablyhas a curable group such as a functional group which cures with ionizingradiation or a thermosetting polar group. The polymerizable compound mayhave both of these reactive groups. In comparison with such apolymerizable compound, the polymer has no such reactive groups.

The polymerizable compound having an ionizing radiation-curablefunctional group may be a fluorine-containing monomer having anethylenic unsaturated bond. Specific examples thereof includefluoroolefins (e.g. fluoroethylene, vinylidene fluoride,tetrafluoroethylene, hexafluoropropylene, perfluorobutadiene,perfluoro-2,2-dimethyl-1,3-dioxole). Examples of those having a(meth)acryloyloxy group include: (meth)acrylate compounds each having afluorine atom in the molecule such as 2,2,2-trifluoroethyl(meth)acrylate, 2,2,3,3,3-pentafluoropropyl (meth)acrylate,2-(perfluorobutyl)ethyl (meth)acrylate, 2-(perfluorohexyl)ethyl(meth)acrylate, 2-(perfluorooctyl)ethyl (meth)acrylate,2-(perfluorodecyl)ethyl (meth)acrylate, α-trifluoromethyl methacrylate,and α-trifluoroethyl methacrylate; and fluorine-containingmulti-functional (meth)acrylate compounds each having a C1-C14fluoroalkyl group, fluorocycloalkyl group, or fluoroalkylene grouphaving at least three fluorine atoms and at least two (meth)acryloyloxygroups in the molecule.

The thermosetting polar group is preferably a hydrogen bond-formablegroup such as a hydroxy group, a carboxyl group, an amino group, or anepoxy group. These groups are excellent not only in adhesion with thecoating but also in affinity with inorganic ultra-fine particles such assilica. Examples of the polymerizable compound having a thermosettingpolar group include 4-fluoroethylene/perfluoroalkyl vinyl ethercopolymers; fluoroethylene/hydrocarbon-type vinyl ether copolymers; andfluorine-modified products of resins such as epoxy, polyurethane,cellulose, phenol, and polyimide.

Examples of the polymerizable compound having both the ionizingradiation-curable functional group and the thermosetting polar groupinclude partially or fully fluorinated alkyl, alkenyl, and aryl estersof acrylic or methacrylic acid, fully or partially fluorinatedvinylethers, fully or partially fluorinated vinyl esters, and fully orpartially fluorinated vinyl ketones.

Examples of the fluororesin include the following: polymers of monomersor monomer mixtures including at least one fluorine-containing(meth)acrylate compound which is a polymerizable compound having anionizing radiation-curable group; copolymers of at least onefluorine-containing (meth)acrylate compound and a (meth)acrylatecompound having no fluorine atom in its molecule (e.g. methyl(meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate); and monopolymers andcopolymers of fluorine-containing monomers such as fluoroethylene,vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene,3,3,3-trifluoropropylene, 1,1,2-trichloro-3,3,3-trifluoropropylene, andhexafluoropropylene. Silicone-containing vinylidene fluoride copolymersmay also be used which are prepared by blending any of the abovecopolymers with a silicone component. Examples of the silicone componentin this case include (poly)dimethylsiloxane, (poly)diethylsiloxane,(poly)diphenylsiloxane, (poly)methylphenylsiloxane, alkyl-modified(poly)dimethylsiloxane, azo group-containing (poly)dimethylsiloxane,dimethylsilicone, phenylmethylsilicone, alkyl/aralkyl-modified silicone,fluorosilicone, polyether-modified silicone, fatty acid ester-modifiedsilicone, methyl hydrogen silicone, silanol group-containing silicone,alkoxy group-containing silicone, phenol group-containing silicone,methacryl-modified silicone, acryl-modified silicone, amino-modifiedsilicone, carboxylic acid-modified silicone, carbinol-modified silicone,epoxy-modified silicone, mercapto-modified silicone, fluorine-modifiedsilicone, and polyether-modified silicone. Preferable are those having adimethylsiloxane structure.

The fluororesin may be a non-polymer or polymer formed from any of thefollowing compounds. In other words, the fluororesin may be a compoundobtained by reacting a fluorine-containing compound having at least oneisocyanato group in its molecule and a compound having at least onefunctional group reactive with the isocyanato group (e.g. amino group,hydroxy group, carboxyl group) in its molecule; or a compound obtainedby reacting a fluorine-containing polyol (e.g. fluorine-containingpolyether polyol, fluorine-containing alkyl polyol, fluorine-containingpolyester polyol, fluorine-containing ε-caprolactone-modified polyol)and a compound having an isocyanato group.

In addition to the above fluorine atom-containing polymerizablecompounds and polymers, the aforementioned binder resin may be used inadmixture. Further, a curing agent for curing, for example, the reactivegroups and any additives and solvents for improving a coating propertyand for imparting an antifouling property may also be used.

The low refractive layer is preferably formed such that the compositionfor a low refractive layer which is formed from a low refractive indexagent, resin, and the like components has a viscosity of 0.5 to 5 mPa·s(25° C.), and preferably 0.7 to 3 mPa·s (25° C.), for a favorablecoating property. Such a configuration enables to provide an excellentantireflection layer against visible light, to form a uniform thin filmwithout uneven application, and to form a low refractive layerespecially with excellent adhesion.

The resin may be cured by the same method as the hard coat layer to bementioned later. In the case of curing by heat application, thefluororesin composition preferably contains a thermal polymerizationinitiator which, for example, generates a radical by heating to initiatepolymerization of a polymerizable compound.

The optical layered body of the present invention may be produced asfollows. For example, the polyester substrate prepared by the abovemethod is covered by a coating for a hard coat layer; the coating isdried if necessary; and then the coating for a hard coat layer is curedto form a hard coat layer. Subsequently, the low refractive layer isformed on the hard coat layer by a known method as appropriate. Thisprovides the optical layered body of the present invention.

The coating for a hard coat layer may be dried by any method. Ingeneral, the coating is preferably dried at 30° C. to 120° C. for 3 to120 seconds.

The method for curing the coating for a hard coat layer may be any knownmethod appropriately selected depending on the constitutionalcomponents. For example, a coating containing an ultraviolet-curablebinder resin component is cured by ultraviolet radiation onto thecoating.

The ultraviolet radiation is preferably such that the ultravioletradiation dose is 80 mJ/cm² or higher, more preferably 100 mJ/cm² orhigher, and still more preferably 130 mJ/cm² or higher.

The optical layered body of the present invention preferably has aprimer layer between the light-transmitting substrate and the opticalfunctional layer.

The primer layer is disposed for the primary purpose of improving theadhesion between the polyester substrate and the hard coat layer. Inorder to prevent interference fringes due to formation of the primerlayer, the thickness of the primer layer is preferably appropriatelyselected as follows on the basis of the relationship among therefractive indexes (nx and ny) of the light-transmitting substrate, therefractive index (nf) of the optical functional layer, and therefractive index (np) of the primer layer:

(1) the thickness of the primer layer is preferably 3 to 30 nm, providedthat the refractive index (np) of the primer layer is greater than therefractive index (nx) in the slow axis direction of thelight-transmitting substrate and is greater than the refractive index(nf) of the optical functional layer (np>nx and np>nf) or that therefractive index (np) of the primer layer is smaller than the refractiveindex (ny) in the fast axis direction of the light-transmittingsubstrate and is smaller than the refractive index (nf) of the opticalfunctional layer (np<ny and np<nf);

(2) the thickness of the primer layer is preferably 65 to 125 nm,provided that the refractive index (np) of the primer layer is greaterthan the refractive index (nx) in the slow axis direction of thelight-transmitting substrate but is smaller than the refractive index(nf) of the optical functional layer (nx<np<nf) or that the refractiveindex (np) of the primer layer is smaller than the refractive index (ny)in the fast axis direction of the light-transmitting substrate but isgreater than the refractive index (nf) of the optical functional layer(nf<np<ny); or

(3) the thickness of the primer layer is not particularly limited fromthe viewpoint of preventing interference fringes provided that therefractive index (np) of the primer layer is between the refractiveindex (ny) in the fast axis direction of the light-transmittingsubstrate and the refractive index (nx) in the slow axis direction ofthe light-transmitting substrate (ny<np<nx). In order to decrease theamount of light reflected on the interface between the primer layer andthe light-transmitting substrate to weaken interference fringes, therefractive index (np) of the primer layer is preferably as close to thevalue of (nx+ny)/2 as possible.

The following will describe the reason why the above thicknesses of theprimer layers in the situations (1) and (2) are preferred. In thesituation (1), the interface (interface A) between the primer layer andthe optical functional layer and the interface (interface B) between thelight-transmitting substrate and the primer layer show oppositerelationship about the degree of change in the refractive index againstincident natural light. Thus, the natural light reflected on one of theinterface A and the interface B shows free-end reflection, whereas thaton the other interface shows fixed-end reflection; that is, the phasesare reversed. As a result, a thin primer layer allows the light beamsreflected on the respective interfaces to interfere with each other,thereby decreasing their intensities.

In the situation (2), the interface A and the interface B show the samedegree of change in the refractive index and thus the light beamsreflected on the interface A and the interface B have the same phase.Thus, the primer layer with an optical thickness of ¼ the lightwavelength allows the light beams reflected on the respective interfacesto interfere with each other, thereby decreasing their intensities.Since the refractive index of the primer layer is generally about 1.47to about 1.63 as will be mentioned later, the thickness of the primerlayer in the situation (2) is a value calculated on the basis of therefractive index of 1.55, which is the intermediate value between theabove range, and a light wavelength of 380 to 780 nm.

In the case where the difference in refractive index between the primerlayer and the light-transmitting substrate is identical to thedifference in refractive index between the primer layer and the opticalfunctional layer, the reflectances on the respective interfaces are alsoidentical to each other, showing the best effects owing to theinterference in the situations (1) and (2).

The primer layer is preferably 3 to 30 nm in the situation (1). Theprimer layer with a thickness of lower than 3 nm may provideinsufficient adhesion between the polyester substrate and the hard coatlayer. The primer layer with a thickness exceeding 30 nm may provide aninsufficient property of preventing interference fringes on the opticallayered body of the present invention. The lower limit of the thicknessof the primer layer in the situation (1) is more preferably 10 nm,whereas the upper limit thereof is more preferably 20 nm.

The primer layer is preferably 65 to 125 nm in thickness in thesituation (2). The primer layer with a thickness beyond this range mayprovide an insufficient property of preventing interference fringes onthe optical layered body of the present invention. The lower limit ofthe thickness of the primer layer in the situation (2) is morepreferably 70 nm, whereas the upper limit thereof is more preferably 110nm.

The primer layer may have any thickness without limitation in thesituation (3). The lower limit of the thickness is preferably 3 nm,whereas the upper limit thereof is preferably 125 nm.

The thickness of the primer layer is an average value (nm) of, forexample, arbitrarily selected 10 points in the cross section observedthrough an electron microscope (SEM, TEM, STEM). For a very thin primerlayer, the cross section observed at a high magnification is recorded asa picture and this picture is further magnified to measure thethickness. The magnification turns a layer-interface line, which is verythin enough to clarify the boundary line, into a thick one. In thiscase, the width of this thick line is bisected and the center portioncrossing the width serves as the boundary line for the measurement.

The material of such a primer layer is not particularly limited as longas it has adhesiveness with the light-transmitting substrate. Thematerial may be one conventionally used as a primer layer of an opticallayered body.

In consideration of conventional materials of a primer layer for aconventional optical layered body, those satisfying favorableadhesiveness and hardness provide a primer layer with a refractive indexof 1.47 to 1.63. In comparison with the case where the thickness of theprimer layer is not adjusted, the optical layered body of the presentinvention favorably makes it possible to select the material of theprimer layer within a very wide range of materials.

For the refractive index (nf) of the optical functional layer in thesituations (1) and (2), the difference in refractive index between theprimer layer and the light-transmitting substrate is preferably as closeto the difference in refractive index between the primer layer and theoptical functional layer as possible for the best effects owing to theinterference. In the situation (3), the refractive index (nf) ispreferably as close to the refractive index of the primer layer aspossible for prevention of an increase in the interface.

The primer layer in the optical layered body of the present inventionmay be formed from a composition for a primer layer. The composition fora primer layer is prepared by mixing and dispersing the aforementionedmaterials and, if necessary, a photo-polymerization initiator and othercomponents in a solvent.

The mixing and dispersing may be preferably achieved using a knowndevice such as a paint shaker, a bead mill, or a kneader.

The solvent is preferably water, and the composition for a primer ispreferably used in the form of an aqueous coating liquid such as anaqueous solution, an aqueous dispersion, or an emulsion. A small amountof an organic solvent may also be used together.

Examples of the organic solvent include: alcohols (e.g. methanol,ethanol, propanol, isopropanol, n-butanol, s-butanol, t-butanol, benzylalcohol, PGME, ethylene glycol); ketones (e.g. acetone, methyl ethylketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone,heptanone, diisobutyl ketone, diethyl ketone); aliphatic hydrocarbons(e.g. hexane, cyclohexane); halogenated hydrocarbons (e.g. methylenechloride, chloroform, carbon tetrachloride); aromatic hydrocarbons (e.g.benzene, toluene, xylene); amides (e.g. dimethyl formamide, dimethylacetamide, n-methylpyrrolidone); ethers (e.g. diethyl ether, dioxane,tetrahydrofuran); ether alcohols (e.g. 1-methoxy-2-propanol); and esters(e.g. methyl acetate, ethyl acetate, butyl acetate, isopropyl acetate).

The other components are not particularly limited. Examples thereofinclude leveling agents, organic and inorganic fine particles,photo-polymerization initiators, thermal polymerization initiators,cross-linking agents, curing agents, polymerization accelerators,viscosity modifiers, antistatic agents, antioxidants, antifoulingagents, slipping agents, refractive index modifiers, and dispersingagents.

The composition for a primer layer preferably satisfies a total solidscontent of 3 to 20%. A total solids content of less than 3% may causeremaining of the solvent and whitening. A total solids content exceeding20% may cause the composition for a primer layer to have a highviscosity and a poor coating property. Thereby, the coating may sufferunevenness or stripes on its surface and may fail to achieve a desiredthickness. The solids content is more preferably 4 to 10%.

The composition for a primer layer may be applied onto the polyestersubstrate at any stage of the process. It is preferably applied duringthe production of the polyester substrate, and more preferably appliedonto the polyester substrate before oriented crystallization.

The “polyester substrate before oriented crystallization” includesunstretched films, monoaxially oriented films prepared by orienting theunstretched film along the machine direction or the transversedirection, films oriented along the two direction, the machine directionand the transverse direction, at a low stretching ratio (biaxiallystretched film before completion of the oriented crystallization byre-stretching along the machine direction and the transverse direction),and the like. In particular, it is preferable to apply an aqueouscoating liquid of the composition for a primer layer onto an unstretchedfilm or a uniaxially stretched film that is oriented along one directionand then perform machine direction stretching and/or transversedirection stretching and heat fixing.

The application of the composition for a primer layer onto the polyestersubstrate preferably follows a preliminary treatment on the surface ofthe polyester substrate for improving the coating property, such asphysical treatments including corona treatment, flame treatment, andplasma treatment. Alternatively, the composition for a primer layer ispreferably used with a surfactant that is chemically inactive with thecomposition.

The composition for a primer layer may be applied by any knownapplication method. Examples thereof include roll coating, gravurecoating, roll brushing, spray coating, air knife coating, immersion, andcurtain coating. These techniques may be performed alone or incombination. The film may be formed on one side of the polyestersubstrate or on both sides thereof as appropriate.

As mentioned above, the primer layer having a refractive index and athickness each within the above specific range enables to exert itseffect of preventing interference fringes.

Such a primer layer and hard coat layer whose refractive indexes satisfythe specific relationship are each preferably formed from a compositionthat has an adjusted refractive index prepared by blending thecomposition for a hard coat layer or the composition for a primer layercontaining high refractive fine particles and/or low refractive fineparticles.

The high refractive fine particles may suitably be metal oxide fineparticles whose refractive index is 1.50 to 2.80, for example. Specificexamples of the metal oxide fine particles include titanium oxide (TiO₂,refractive index: 2.71), zirconium oxide (ZrO₂, refractive index: 2.10),cerium oxide (CeO₂, refractive index: 2.20), tin dioxide (SnO₂,refractive index: 2.00), antimony tin oxide (ATO, refractive index: 1.75to 1.95), indium tin oxide (ITO, refractive index: 1.95 to 2.00),phosphorus tin oxide (PTO, refractive index: 1.75 to 1.85), antimonypentoxide (Sb₂O₅, refractive index: 2.04), aluminum zinc oxide (AZO,refractive index: 1.90 to 2.00), gallium zinc oxide (GZO, refractiveindex: 1.90 to 2.00), and zinc antimonate (ZnSb₂O₆, refractive index:1.90 to 2.00). In particular, tin dioxide (SnO₂), antimony tin oxide(ATO), indium tin oxide (ITO), phosphorus tin compound (PTO), antimonypentoxide (Sb₂O₅), aluminum zinc oxide (AZO), gallium zinc oxide (GZO),and zinc antimonate (ZnSb₂O₆) are advantageous in that they areconductive metal oxides, and enable to control the state of dispersionof fine particles and form a conductive path to impart antistaticproperties.

The low refractive fine particles may suitably be those having arefractive index of 1.20 to 1.45. Such low refractive fine particles maybe fine particles used for a conventionally known low refractive layer,and examples thereof include the aforementioned hollow silica particlesand fine particles of metal fluoride such as LiF (refractive index:1.39), MgF₂ (magnesium fluoride, refractive index: 1.38), AlF₃(refractive index: 1.38), Na₃AlF₆ (cryolite, refractive index: 1.33),and NaMgF₃ (refractive index: 1.36).

The amounts of the high refractive fine particles and the low refractivefine particles are not particularly limited. For example, the amountsthereof may be appropriately adjusted such that the refractive index ofa hard coat layer to be formed satisfies the aforementioned relationshipin terms of a weighted mean with a previously measured refractive indexof cured products of the resin components added to the composition for ahard coat layer.

The hard coat layer may be completed as follows: applying thecomposition for a hard coat layer onto the primer layer formed by theabove method to form a coating for a hard coat layer; if necessary,drying the coating; and then curing the coating for a hard coat layer.

In the case where the composition for a hard coat layer contains anultraviolet-curable resin, the composition for a primer layer maycontain an initiator used for curing the coating for a hard coat layerso as to assure further adhesion between the hard coat layer and theprimer layer.

The optical layered body of the present invention preferably has ahardness of HB or higher, and more preferably H or higher in the pencilhardness test (load: 4.9 N) in conformity with JIS K5600-5-4 (1999).

The optical layered body of the present invention preferably has a totalluminous transmittance of 80% or higher. The optical layered body with atotal luminous transmittance of lower than 80% may deteriorate colorreproducibility and visibility when attached to an image display device,and may fail to provide a desired contrast. The total luminoustransmittance is more preferably 90% or higher.

The total luminous transmittance can be measured using a haze meter(MURAKAMI COLOR RESEARCH LABORATORY CO, Ltd., product No.: HM-150) bythe method in conformity with JIS K7361.

The optical layered body of the present invention preferably has a hazeof 1% or lower. The optical layered body with a haze exceeding 1% mayfail to provide desired optical characteristics and may deterioratevisibility when attached to the image display device.

The haze can be measured using a haze meter (MURAKAMI COLOR RESEARCHLABORATORY CO, Ltd., product No.: HM-150) by the method in conformitywith JIS K7136.

In the case where the optical functional layer is a hard coat layer, theoptical layered body of the present invention may be produced by, forexample, forming a hard coat layer from the composition for a hard coatlayer on the light-transmitting substrate. In the case where the opticalfunctional layer has a low refractive layer disposed on the hard coatlayer, the optical layered body of the present invention may be producedby, for example, forming a hard coat layer from the composition for ahard coat layer on the light-transmitting substrate, and then forming alow refractive layer from the composition for a low refractive layer onthe hard coat layer.

The composition for a hard coat layer, the method for forming a hardcoat layer, the composition for a low refractive layer, and the methodfor forming a low refractive layer may be the same materials and themethods as mentioned above.

Another aspect of the present invention relates to a polarizer that isconfigured to be disposed on a surface of an image display device, thepolarizer comprising: a polarizing element; and an optical layered bodydisposed on the polarizing element, the optical layered body including alight-transmitting substrate having in-plane birefringence and anoptical functional layer disposed on one surface of thelight-transmitting substrate, the light-transmitting substrate having aslow axis with a greater refractive index and the polarizing elementhaving an absorption axis, the light-transmitting substrate and thepolarizing element being disposed such that the slow axis of thelight-transmitting substrate and the absorption axis of the polarizingelement are orthogonal to each other, and the polarizer being configuredto be disposed on a display screen of the image display device such thatthe slow axis of the light-transmitting substrate is in parallel withthe vertical direction of the display screen.

The optical layered body in the polarizer of the present invention maybe the same as the optical layered body of the present invention.

For the same reasons as in the optical layered body of the presentinvention, the polarizer of the present invention preferably satisfiesthat the light-transmitting substrate having in-plane birefringence hasa retardation of 3000 nm or greater and that the difference (nx−ny) is0.05 or greater, where nx represents the refractive index in the slowaxis direction, which is the direction showing a greater refractiveindex, and ny represents the refractive index in the fast axis directionthat is orthogonal to the slow axis direction.

For the same reasons as in the optical layered body of the presentinvention, the polarizer of the present invention preferably has aprimer layer between the light-transmitting substrate and the opticalfunctional layer and the primer layer preferably has a thicknessappropriately selected in accordance with the above situations (1) to(3).

The polarizing element is not particularly limited. Examples thereofinclude polyvinyl alcohol films, polyvinyl formal films, polyvinylacetal films, and ethylene/vinyl acetate copolymerized saponified films,each of which are dyed using, for example, iodine and then stretched.The lamination treatment of the polarizing element and the opticallayered body preferably include a step of saponifying thelight-transmitting substrate. The saponification provides good adhesionand antistatic effects.

The polarizer of the present invention satisfies that thelight-transmitting substrate and the polarizing element are disposedsuch that the slow axis extending along the direction showing a greaterrefractive index of the light-transmitting substrate is orthogonal tothe absorption axis of the polarizing element. Since the polarizer ofthe present invention satisfies that the light-transmitting substrateand the polarizing element are disposed as mentioned above and disposedsuch that the slow axis extending along the direction showing a greaterrefractive index of the light-transmitting substrate is in parallel withthe vertical direction of a display screen of the image display device,the polarizer is excellent in anti-reflection properties andbright-field contrast similarly to the aforementioned optical layeredbody of the present invention.

The phrase “the light-transmitting substrate and the polarizing elementare disposed such that the slow axis extending along the directionshowing a greater refractive index of the light-transmitting substrateis orthogonal to the absorption axis of the polarizing element” meansthe state that the light-transmitting substrate and the polarizingelement are disposed such that the angle formed between the slow axis ofthe light-transmitting substrate and the absorption axis of thepolarizing element is within 90°±40°.

The polarizer of the present invention preferably satisfies that theangle between the slow axis of the light-transmitting substrate and theabsorption axis of the polarizing element is 90°±30°, more preferably90°±10°, and still more preferably 90°±5°. The polarizer of the presentinvention with an angle between the slow axis of the light-transmittingsubstrate and the absorption axis of the polarizing element of 90°±40°enables to improve the anti-reflection properties and the bright-fieldcontrast. The angle between the slow axis of the light-transmittingsubstrate and the absorption axis of the polarizing element in thepolarizer of the present invention is most preferably 90° for thepurpose of improving the anti-reflection properties and the bright-fieldcontrast. Thus, the angle between the slow axis of thelight-transmitting substrate and the absorption axis of the polarizingelement is preferably not 90°±40° but 90°±30°, and more preferably90°±10°. Further, the angle between the slow axis of thelight-transmitting substrate and the absorption axis of the polarizingelement is still more preferably 0°±5° because such an angle enables toraise the anti-reflection properties and the bright-field contrast tothe same levels as the angle of 0°.

Such a polarizer of the present invention may be produced by disposingthe light-transmitting substrate of the optical layered body and thepolarizing element such that the slow axis extending along the directionshowing a greater refractive index of the light-transmitting substrateis orthogonal to the absorption axis of the polarizing element. In thiscase, the slow axis extending along the direction showing a greaterrefractive index of the light-transmitting substrate is in parallel withthe vertical direction of a display screen of the image display device.

Another aspect of the present invention relates to an image displaydevice comprising the optical layered body of the present invention orthe polarizer of the present invention.

The image display device of the present invention may be any of imagedisplay devices such as LCDs, PDPs, FEDs, ELDs (organic ELDs, inorganicELDs), CRTs, tablet PCs, touchscreens, and electronic paper.

An LCD, which is one representative example of the above image displaydevices, comprises a transmissive display and a light source device thatirradiates the transmissive display from the back side. The imagedisplay device of the present invention which is an LCD comprises thetransmissive display and the optical layered body of the presentinvention or the polarizer of the present invention disposed on thesurface of the transmissive display.

In the case where the image display device of the present invention is aliquid crystal display device comprising the optical layered body of thepresent invention or the polarizer of the present invention, the lightsource device illuminates the optical layered body of the presentinvention or the polarizer of the present invention from the lower side.A retardation film may be disposed between the liquid crystal displayelement and the polarizer of the present invention. This liquid crystaldisplay device may have an adhesive layer between the respective layers,if necessary.

The PDP comprises a front glass substrate having an electrode on itssurface and a back glass substrate that is opposite to the front glasssubstrate and that has an electrode and minute grooves on its surface,wherein a discharge gas is filled into the space between the glasssubstrates, and the grooves each have a red, green, or blue fluorescentlayer. The image display device of the present invention which is a PDPcomprises the optical layered body of the present invention on thesurface of the front glass substrate or a substrate (glass substrate ora film substrate) disposed in front of the front glass substrate.

The image display device of the present invention may be, for example,an ELD device that comprises a glass substrate and luminous bodies (e.g.zinc sulfide, diamine substances), which emit light when a voltage isapplied, deposited on the glass substrate and that shows an image bycontrolling the voltage to be applied onto the substrate, or a CRT thatconverts electric signals into light to show an image visible to thehuman eyes. In this case, the display device has the above opticallayered body of the present invention on the outermost surface or on thesurface of a substrate in front of the outermost surface of the displaydevice.

In the case where the image display device of the present invention is aliquid crystal display device having the optical layered body, thebacklight light source of the liquid crystal display device is notparticularly limited, and is preferably a white-light-emitting diode(white LED). The image display device of the present invention ispreferably a VA-mode or IPS-mode liquid crystal display devicecomprising a white-light-emitting diode as a backlight light source.

The white LED is a white-color-emitting element which is of afluorescent substance type, in other words, which comprises a blue- orultraviolet-light-emitting diode using a compound semiconductor and afluorescent substance in combination. In particular, awhite-light-emitting diode of a light-emitting element that comprises ablue-light-emitting diode utilizing a compound semiconductor and ayttrium/aluminum/garnet-type yellow fluorescent substance in combinationhas a continuous and wide emission spectrum so that it is effective toimprove the anti-reflection properties and the bright-field contrast.Further, such a diode is excellent in luminous efficacy. Thus, such awhite-light-emitting diode is suitable as the backlight light source inthe present invention. In addition, a white LED with a low electricenergy consumption can be widely used, and thus the effect of low energyconsumption can be achieved.

The VA (vertical alignment) mode is an operation mode in which theliquid crystal molecules are aligned perpendicular to the substrate ofthe liquid crystal cell with no voltage application to display a darkscreen, whereas the liquid crystal molecules are tilted with voltageapplication to show a bright screen.

The IPS (in-plane switching) mode is a mode in which one substrate ofthe liquid crystal cell has a comb-shaped electrode pair and atransverse electric field applied to the electrode pair rotates theliquid crystal in the substrate plane to show an image.

The following will describe the reasons why the image display devicecomprising the optical layered body or polarizer of the presentinvention is preferably a VA mode or IPS mode display device with awhite-light-emitting diode as the backlight light source.

The image display device of the present invention is capable of reducingreflection of light beams vibrating along the horizontal direction(S-polarized light), which occupy a large proportion of the lightincident on the display screen, on the optical layered body or thepolarizer of the present invention. As a result, many S-polarized lightbeams pass through the screen. Such S-polarized light passed isgenerally absorbed in the display device, but slight part of the lightreturns to an observer. In the VA mode or IPS mode display device, thepolarizer is disposed closer to the observer than the liquid crystalcell and it has an absorption axis extending in the horizontal directionrelative to the display screen. Thus, the polarizer enables to absorbthe S-polarized light which has passed through the optical layered bodyor polarizer of the present invention, and thereby to reduce the lightreturning to the observer.

The image display device of the present invention may be an organic ELdisplay device comprising a polarizing element disposed on a displayscreen such that the absorption axis of the polarizing element extendsin the horizontal direction relative to the display screen. The organicEL display device requires no polarizing element based on its imagedisplaying principle. Nevertheless, in order to prevent natural lightreflection, the device may have a layered structure of a polarizingelement, a λ/4 retardation film, and an organic EL element from theobserver side. In this case, the polarizing element and the λ/4retardation film each serve as a circular polarizer for preventingnatural light reflection. Here, a common λ/4 retardation film serves asa λ/4 retardation film against only a specific wavelength, and thus itcannot prevent reflection of all the light beams incident on thedisplay. Thus, the absorption axis of the polarizing element extendingin the horizontal direction relative to the display screen enables toabsorb the S-polarized light entering the display screen and reduce thelight entering the inside of the image, and thereby reduce the lightreturning to the observer.

The organic EL display device may employ any of the image display modessuch as a color filter mode in which a white luminous layer is used andthe light passes through a color filter to achieve colored screen; acolor conversion mode in which a blue luminous layer is used and part ofthe light passes through a color-conversion layer to achieve coloredscreen; a three-color mode in which red, green, and blue luminous layersare used; and a combination of this three-color mode with a colorfilter. The material of each luminous layer may be alow-molecular-weight molecule or may be a high-molecular-weightmolecule.

In either case, the image display device of the present invention may beused for image displaying on televisions, computers, electronic paper,touchscreens, tablet PC, and the like. In particular, the image displaydevice of the present invention may be suitably used for any displaysfor high-definition images such as CRTs, liquid crystal panels, PDPs,ELDs, FEDs, and touchscreens.

Another aspect of the present invention relates to a method forproducing an image display device that has an optical layered bodyincluding a light-transmitting substrate having in-plane birefringenceand an optical functional layer disposed on one surface of thesubstrate.

In other words, the method for producing an image display device of thepresent invention is a method for producing an image display device thathas an optical layered body including a light-transmitting substratehaving in-plane birefringence and an optical functional layer disposedon one surface of the substrate. The method comprises the step ofdisposing the optical layered body such that the slow axis extendingalong the direction showing a greater refractive index of thelight-transmitting substrate is in parallel with the vertical directionof a display screen of the image display device.

The optical layered body in the method for producing an image displaydevice of the present invention may be the same as the aforementionedoptical layered body of the present invention.

The phrase “disposing the optical layered body such that the slow axisextending along the direction showing a greater refractive index of thelight-transmitting substrate is in parallel with the vertical directionof a display screen of the image display device” means that the opticallayered body is disposed such that the angle between the slow axis andthe vertical direction of the display screen is within 0°±40°.

As is mentioned for the optical layered body of the present invention,the angle between the slow axis of the light-transmitting substrate andthe vertical direction of the display screen is preferably 0°±30°, morepreferably 0°±10°, and still more preferably 0°±5°.

The aforementioned image display device of the present invention isexcellent in anti-reflection properties and bright-field contrast, andhas improved visibility. Such a method for improving visibility by theimage display device of the present invention is also one aspect of thepresent invention.

In other words, the method for improving visibility of an image displaydevice of the present invention is a method for improving visibility ofan image display device that has an optical layered body including alight-transmitting substrate having in-plane birefringence and anoptical functional layer disposed on one surface of the substrate. Themethod comprises the step of disposing the optical layered body suchthat the slow axis extending along the direction showing a greaterrefractive index of the light-transmitting substrate is in parallel withthe vertical direction of a display screen of the image display device.

The optical layered body in the method for improving visibility of animage display device of the present invention may be the same as theaforementioned optical layered body of the present invention. The imagedisplay device therein may be the same as the aforementioned imagedisplay device of the present invention.

The phrase “disposing the optical layered body such that the slow axisextending along the direction showing a greater refractive index of thelight-transmitting substrate is in parallel with the vertical directionof a display screen of the image display device” means that the opticallayered body is disposed such that the angle between the slow axis andthe vertical direction of a display screen is within 0°±40°.

As is mentioned for the polarizer of the present invention, the anglebetween the slow axis of the light-transmitting substrate and thevertical direction of a display screen is preferably 0°±30°, morepreferably 0°±10°, and still more preferably 0°±5°.

Advantageous Effects of Invention

Since the optical layered body and polarizer of the present inventionhave the aforementioned structures, even a light-transmitting substratehaving in-plane birefringence such as a polyester film can contribute toproduction of an image display device excellent in anti-reflectionproperties and bright-field contrast.

Thus, the optical layered body and polarizer of the present inventioncan be suitably used for cathode ray tube display devices (CRTs), liquidcrystal displays (LCDs), plasma display panels (FDPs),electroluminescence displays (ELDs), field emission displays (FEDs),electronic paper, touchscreens, tablet PCs, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the spectrum of a backlight light source of aliquid crystal monitor used in, for example, the following examples.

DESCRIPTION OF EMBODIMENTS

(Evaluation of Bright-Field Contrast)

An optical layered body was disposed on a polarizing element of a liquidcrystal monitor (FLATORON IPS226V (LG Electronics Japan)) at theobserver side such that the optical functional layer was at the observerside so that the S-polarized light and the fast axis of thelight-transmitting substrate showed the same relationship as in thereflectance measurement. The bright-field contrast of the display screenwas visually observed at an ambient illumination intensity of 400 lx (inthe bright field).

Specifically, the bright-field contrast is represented by the followingformula, and generally the rate of change in bright-field whiteluminance is low but the rate of change in bright-field black luminanceis high. Thus, the bright-field black luminance has an influence on thebright-field contrast. Further, the intrinsic black luminance of a panelis smaller than the bright-field black luminance and thus is eliminated.Accordingly, the blackness (bright-field black luminance) was evaluatedas follows and this evaluation was substantially defined as theevaluation on the bright-field contrast.

Two liquid crystal monitors were prepared with different angles betweenthe S-polarized light and the fast axis of the light-transmittingsubstrate (one is referred to as a liquid crystal monitor A and theother as a liquid crystal monitor B). The liquid crystal monitors A andB were placed side by side, and were subjected to a sensory test by 15subjects (the subjects observed the liquid crystal monitors with a blackscreen from the position apart from the display by 50 to 60 cm andevaluated which screen was blacker). The liquid crystal monitor that 12or more subjects evaluated blacker was evaluated as good in bright-fieldcontrast, whereas the monitor that less than 12 subjects, in otherwords, 11 or less subjects evaluated blacker was evaluated as poor. Theangles of disposing the optical layered bodies on the liquid crystalmonitors A and B were appropriately adjusted for evaluation in therespective examples and comparative examples. The monitor that 13 ormore subjects evaluated blacker was evaluated as particularly better.

Bright-field contrast: CR=LW/LB

Bright-field white luminance (LW): luminance of white screen on displaydevice in bright field with natural light (ambient illuminationintensity 400 lx)

Bright-field black luminance (LB): luminance of black screen on displaydevice in bright field with natural light (ambient illuminationintensity 400 lx)

(Measurement of Reflectance)

A black plastic tape (Yamato Vinyl Tape No 200-38-21, 38 mm width) wasattached onto the measurement side, that is, the side opposite to theoptical functional layer, of the optical layered body. The 5-degreereflectance was measured for each case where the slow axis of thelight-transmitting substrate was in parallel with the S-polarized lightand the case where the fast axis was in parallel therewith (polarizedlight measurement) using a spectrophotometer (automatic absolutereflectance measurement unit V7100-series, VAR-7010, JASCO Corp.).

(Evaluation of Rainbow Interference Pattern)

In each of the examples, comparative examples, and reference examples,an observer observed visually and through polarized sunglasses an imagethat was displayed on the liquid crystal monitor provided with anoptical layered body for the aforementioned evaluation of thebright-field contrast from the front and a diagonal direction (about)50° at the position apart from the monitor by 50 to 60 cm. Thereby, arainbow interference pattern was evaluated.

FIG. 1 shows a spectrum of the backlight light source of the liquidcrystal monitor used.

(Measurement of Retardation)

The retardation of the light-transmitting substrate was measured asfollows.

For a stretched light-transmitting substrate, the direction of thepolarization axis of the light-transmitting substrate was measured usingtwo polarizers. The refractive indexes (nx and ny) against thewavelength of 590 nm of the two axes orthogonal to the polarization axiswere measured using an Abbe refractometer (NAR-4T, ATAGO CO., LTD.). Theaxis that shows a greater refractive index was defined as the slow axis.The thickness d (nm) of the light-transmitting substrate was measuredusing an electric micrometer (ANRITSU CORP.) and its unit was convertedinto nanometer. The product between the difference in refractive indexes(nx−ny) and the film thickness d (nm) provided the retardation.

(Measurement of Refractive Index)

The refractive index was measured using an ellipsometer (UVISEL, HORIBA,Ltd.).

(Confirmation of In-Plane Birefringence)

The presence of in-plane birefringence was confirmed as follows. Thein-plane phase difference was measured using KOBRA-WR (Oji ScientificInstruments) at a measurement angle of 0° and a measurement wavelengthof 589.3 nm. The in-plane phase difference of less than 20 nm wasdefined as the absence of in-plane birefringence, whereas the in-planephase difference of 20 nm or higher was defined as the presence ofin-plane birefringence.

Example 1, Comparative Example 1

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C., stretched at a stretchingratio of 4.5 times and 120° C. in a first direction, and then stretchedat a stretching ratio of 1.5 times and 120° C. in the directionorthogonal to the first stretching direction using a biaxial stretchingtester (Toyo Seiki Seisaku-sho, Ltd.). This yielded a light-transmittingsubstrate (nx=1.70, ny=1.60, (nx−ny)=0.10, thickness=80 val,retardation=8000 nm). Next, pentaerythritol triacrylate (PETA) wasdissolved in an MIBK solvent at 30% by mass and then the solution wasmixed with a photo-polymerization initiator (Irg 184, BASF) so as to be5% by mass for the solids content in the solution, thereby preparing acomposition for an optical functional layer. The composition was appliedon the substrate using a bar coater so as to give a thickness afterdried of 5 μm, thereby producing a coating as an optical functionallayer.

The produced coating was heated for one minute at 70° C. to remove thesolvent. The coat surface was cured by irradiation with ultravioletrays. This yielded an optical layered body comprising an opticalfunctional layer with a refractive index (nf) of 1.53. In Example 1, theoptical layered body was disposed such that the S-polarized light was inparallel with the fast axis of the light-transmitting substrate (theangle between the S-polarized light and the fast axis of thelight-transmitting substrate was 0°), and the reflectance in this casewas 4.45%. In Comparative Example 1, the optical layered body wasdisposed such that the S-polarized light was in parallel with the slowaxis of the light-transmitting substrate (the angle between theS-polarized light and the fast axis of the light-transmitting substratewas 90°), and the reflectance in this case was 4.73%. Therefore, theoptical layered body of Example 1 was better in anti-reflectionproperties.

In order to provide the same relationship between the S-polarized lightand the fast axis of the light-transmitting substrate as in thereflectance measurement, the optical layered body was disposed on thepolarizing element of a liquid crystal monitor (FLATORON IPS226V (LGElectronics Japan)) at the observer side such that the opticalfunctional layer was at the observer side. The bright-field contrast ofthe display screen was visually evaluated at an ambient illuminationintensity of 400 lx (bright field).

Example 1 evaluated the values in the state that the S-polarized lightvibrating in the horizontal direction relative to the display screen,which occupied a greater part of the light incident on the displayscreen, is in parallel with the fast axis of the light-transmittingsubstrate (the slow axis of the light-transmitting substrate is inparallel with the vertical direction of the display screen, in otherwords, the angle between the slow axis of the light-transmittingsubstrate and the vertical direction of a display screen was 0°).Comparative Example 1 evaluated the values in the state that theS-polarized light was in parallel with the slow axis of thelight-transmitting substrate (the angle between the slow axis of thelight-transmitting substrate and the vertical direction of a displayscreen was 90°). As a result, the liquid crystal monitor A using theoptical layered body of Example 1 was particularly better inbright-field contrast of the display screen than the liquid crystalmonitor B of the optical layered body of Comparative Example 1. Theliquid crystal monitor A using the optical layered body of Example 1showed no rainbow interference pattern and the visibility was very wellimproved. In contrast, the liquid crystal monitor B using the opticallayered body of Comparative Example 1 showed no rainbow interferencepattern but was poorer in bright-field contrast and anti-reflectionproperties than the liquid crystal monitor A using the optical layeredbody of Example 1.

Example 2, Comparative Example 2

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C., stretched at a stretchingratio of 4.5 times and 120° C. in a first direction, and then stretchedat a stretching ratio of 1.8 times and 120° C. in the directionorthogonal to the first stretching direction using a′ biaxial stretchingtester (Toyo Seiki Seisaku-sho, Ltd.). This yielded a light-transmittingsubstrate (nx=1.68, ny=1.62, (nx−ny)=0.06, thickness=80 μm,retardation=4800 nm).

Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. Using the obtained optical layered body, the reflectance wasmeasured and the bright-field contrast was evaluated in the same manneras in Example 1 (the angle between the S-polarized light and the fastaxis of the light-transmitting substrate was 0°). As a result, thereflectance of the optical layered body of Example 2 was 4.46%. InComparative Example 2 wherein the S-polarized light was in parallel withthe slow axis of the light-transmitting substrate (the angle between theS-polarized light and the fast axis of the light-transmitting substratewas 90°), the reflectance of the optical layered body was 4.63%.Therefore, the optical layered body of Example 2 was better inanti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 2had a particularly better bright-field contrast of the display screen,which was evaluated in the same manner as in Example 1, than the liquidcrystal monitor B using the optical layered body of Comparative Example2. The liquid crystal monitor A using the optical layered body ofExample 2 showed no rainbow interference pattern and the visibility wasvery well improved. In contrast, the liquid crystal monitor B using theoptical layered body of Comparative Example 2 showed no rainbowinterference pattern but was poorer in bright-field contrast than theliquid crystal monitor A using the optical layered body of Example 2.

Example 3, Comparative Example 3

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C. and stretched at astretching ratio of 4.5 times and 120° C. in one direction using abiaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). On one side ofthe film was uniformly applied a resin composition for a primer layerthat contains an aqueous dispersion of polyester resin (28.0 parts bymass) and water (72.0 parts by mass) using a roll coater. Then, thiscoated film was dried at 95° C. and stretched at a stretching ratio of1.5 times in the direction orthogonal to the former stretchingdirection. This yielded a light-transmitting substrate comprising a film(nx=1.70, ny=1.60, (nx−ny)=0.10, thickness=retardation=8000 nm) and aprimer layer (refractive index=1.56, thickness=100 nm).

Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. Using the obtained optical layered body, the reflectance wasmeasured and the bright-field contrast was evaluated in the same manneras in Example 1 (the angle between the S-polarized light and the fastaxis of the light-transmitting substrate was 0°). As a result, thereflectance of the optical layered body of Example 3 was 4.36%. InComparative Example 3 wherein the S-polarized light was in parallel withthe slow axis of the light-transmitting substrate (the angle between theS-polarized light and the fast axis of the light-transmitting substratewas 90°), the reflectance of the optical layered body was 4.48%.Therefore, the optical layered body of Example 3 was better inanti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 3had a particularly better bright-field contrast of the display screen,which was evaluated in the same manner as in Example 1, than the liquidcrystal monitor B using the optical layered body of Comparative Example3. The liquid crystal monitor A using the optical layered body ofExample 3 showed no rainbow interference pattern and the visibility wasvery well improved. In contrast, the liquid crystal monitor B using theoptical layered body of Comparative Example 3 showed no rainbowinterference pattern but was poorer in bright-field contrast than theliquid crystal monitor A using the optical layered body of Example 3.

Example 4, Comparative Example 4

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C. and stretched at astretching ratio of 4.0 times and 120° C. in one direction using abiaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). On one side ofthe film was uniformly applied a resin composition for a primer layerthat contains an aqueous dispersion of polyester resin (28.0 parts bymass) and water (72.0 parts by mass) using a roll coater. Then, thiscoated film was dried at 95° C. and stretched at a stretching ratio of1.8 times in the direction orthogonal to the former stretchingdirection. This yielded a light-transmitting substrate comprising a film(nx=1.68, ny=1.63, (nx−ny)=0.05, thickness 70 μm, retardation=3500 nm)and a primer layer (refractive index (np)=1.56, thickness=100 nm).

Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. The optical functional layer was formed on the primer layer.Using the obtained optical layered body, the reflectance was measuredand the bright-field contrast was evaluated in the same manner as inExample 1 (the angle between the S-polarized light and the fast axis ofthe light-transmitting substrate was 0°). As a result, the reflectanceof the optical layered body of Example 4 was 4.38%. In ComparativeExample 4 wherein the S-polarized light was in parallel with the slowaxis of the light-transmitting substrate (the angle between theS-polarized light and the fast axis of the light-transmitting substratewas 90°), the reflectance of the optical layered body was 4.47%.Therefore, the optical layered body of Example 4 was better inanti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 4had a particularly better bright-field contrast of the display screen,which was evaluated in the same manner as in Example 1, than the liquidcrystal monitor B using the optical layered body of Comparative Example4. The liquid crystal monitor A using the optical layered body ofExample 4 showed no rainbow interference pattern and the visibility wasvery well improved. In contrast, the liquid crystal monitor B using theoptical layered body of Comparative Example 4 showed no rainbowinterference pattern but was poorer in bright-field contrast than theliquid crystal monitor A using the optical layered body of Example 4.

Example 5, Comparative Example 5

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C. and stretched at astretching ratio of 4.5 times and 120° C. in one direction using abiaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). On one side ofthe film was uniformly applied a resin composition for a primer layerthat contains an aqueous dispersion of polyester resin (28.0 parts bymass) and water (72.0 parts by mass) using a roll coater. Then, thiscoated film was dried at 95° C. and stretched at a stretching ratio of1.5 times in the direction orthogonal to the former stretchingdirection. This yielded a light-transmitting substrate comprising a film(nx=1.70, ny=1.60, (nx−ny)=0.10, thickness=38 μm, retardation=3800 nm)and a primer layer (refractive index (np)=1.56, thickness=100 nm).

Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. The optical functional layer was formed on the primer layer.The obtained optical layered body was disposed such that the anglebetween the S-polarized light and the fast axis of thelight-transmitting substrate was 30°, and the reflectance of the opticallayered body of Example 5 was measured to be 4.39%. On the other hand,the optical layered body was disposed such that the angle between theS-polarized light and the slow axis of the light-transmitting substratewas 30°, and the reflectance of the optical layered body of ComparativeExample 5 was measured to be 4.45%. Therefore, the optical layered bodyof Example 5 was better in anti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 5had a better bright-field contrast of the display screen, which wasevaluated in the same manner as in Example 1, than the liquid crystalmonitor B using the optical layered body of Comparative Example 5. Theliquid crystal monitor A using the optical layered body of Example 5showed a rainbow interference pattern that was slightly observed throughpolarized sunglasses and had no disadvantage in practice, and thus themonitor A had improved visibility. In contrast, the liquid crystalmonitor B using the optical layered body of Comparative Example 5 showeda rainbow interference pattern that was slightly observed throughpolarized sunglasses and had no disadvantage in practice; however, themonitor B was poorer in bright-field contrast than the liquid crystalmonitor A using the optical layered body of Example 5.

For a liquid crystal monitor A′ wherein the value of angle between theS-polarized light and the fast axis of the light-transmitting substratewas the same as that in the optical layered body of Example 5 but theangle had a negative sign, and for a liquid crystal monitor B′ whereinthe value of angle between the S-polarized light and the slow axis ofthe light-transmitting substrate was the same as that in the opticallayered body of Comparative Example 5 but the angle had a negative sign,the reflectance and the bright-field contrast were evaluated. Theseevaluations showed the same results as those of the liquid crystalmonitor A using the optical layered body of Example 5 and the liquidcrystal monitor B using the optical layered body of Comparative Example5.

Example 6, Comparative Example 6

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C. and stretched at astretching ratio of 4.5 times and 120° C. in one direction using abiaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). On one side ofthe film was uniformly applied a resin composition for a primer layerthat contains an aqueous dispersion of polyester resin (28.0 parts bymass) and water (72.0 parts by mass) using a roll coater. Then, thiscoated film was dried at 95° C. and stretched at a stretching ratio of1.5 times in the direction orthogonal to the former stretchingdirection. This yielded a light-transmitting substrate comprising a film(nx=1.70, ny=1.60, (nx−ny)=0.10, thickness=10 μm, retardation=1000 nm)and a primer layer (refractive index (np)=1.56, thickness=100 nm).

Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. The obtained optical layered body was disposed such that theS-polarized light was in parallel with the fast axis of thelight-transmitting substrate (the angle between the S-polarized lightand the fast axis of the light-transmitting substrate was 0°), and thereflectance of the optical layered body of Example 6 was measured to be4.40%. On the other hand, the optical layered body was disposed suchthat the S-polarized light was in parallel with the slow axis (the anglebetween the S-polarized light and the fast axis of thelight-transmitting substrate was 90°), and the reflectance of theoptical layered body of Comparative Example 6 was measured to be 4.47%.Therefore, the optical layered body of Example 6 was better inanti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 6had a better bright-field contrast of the display screen, which wasevaluated in the same manner as in Example 1, than the liquid crystalmonitor B using the optical layered body of Comparative Example 6. Theliquid crystal monitor A using the optical layered body of Example 6showed no rainbow interference pattern and the visibility was improved.In contrast, the liquid crystal monitor B using the optical layered bodyof Comparative Example 6 showed no rainbow interference pattern but waspoorer in bright-field contrast than the liquid crystal monitor A usingthe optical layered body of Example 6.

Example 7, Comparative Example 7

The optical layered body obtained in Example 1 was disposed such thatthe angle between the S-polarized light and the fast axis of thelight-transmitting substrate was 5°, and the reflectance of the opticallayered body of Example 7 was measured to be 4.46%. On the other hand,the optical layered body was disposed such that the angle between theS-polarized light and the slow axis of the light-transmitting substratewas 5°, and the reflectance of the optical layered body of ComparativeExample 7 was measured to be 4.72%. Therefore, the optical layered bodyof Example 7 was better in anti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 7had a particularly better bright-field contrast of the display screen,which was evaluated in the same manner as in Example 1, than the liquidcrystal monitor B using the optical layered body of Comparative Example7. The liquid, crystal monitor A using the optical layered body ofExample 7 showed no rainbow interference pattern and thus the visibilitywas very well improved. In contrast, the liquid crystal monitor B usingthe optical layered body of Comparative Example 7 showed no rainbowinterference pattern but was poorer in bright-field contrast andantireflection properties than the liquid crystal monitor A using theoptical layered body of Example 7.

For a liquid crystal monitor A′ wherein the value of angle between theS-polarized light and the fast axis of the light-transmitting substratewas the same as that in the optical layered body of Example 7 but theangle had a negative sign, and for a liquid crystal monitor B′ whereinthe value of angle between the S-polarized light and the slow axis ofthe light-transmitting substrate was the same as that in the opticallayered body of Comparative Example 7 but the angle had a negative sign,the reflectance and the bright-field contrast were evaluated. Theseevaluations showed the same results as those of the liquid crystalmonitor A using the optical layered body of Example 7 and the liquidcrystal monitor B using the optical layered body of Comparative Example7.

Example 8, Comparative Example 8

The optical layered body obtained in Example 1 was disposed such thatthe angle between the S-polarized light and the fast axis of thelight-transmitting substrate was 10°, and the reflectance of the opticallayered body of Example 8 was measured to be 4.48%. On the other hand,the optical layered body was disposed such that the angle between theS-polarized light and the slow axis of the light-transmitting substratewas 10°, and the reflectance of the optical layered body of ComparativeExample 8 was measured to be 4.68%. Therefore, the optical layered bodyof Example 8 was better in anti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 8had a particularly better bright-field contrast of the display screen,which was evaluated in the same manner as in Example 1, than the liquidcrystal monitor B using the optical layered body of Comparative Example8. The liquid crystal monitor A using the optical layered body ofExample 8 showed no rainbow interference pattern and the visibility wasvery well improved. In contrast, the liquid crystal monitor B using theoptical layered body of Comparative Example 8 showed no rainbowinterference pattern but was poorer in bright-field contrast andantireflection properties than the liquid crystal monitor A using theoptical layered body of Example 8.

For a liquid crystal monitor A′ wherein the value of angle between theS-polarized light and the fast axis of the light-transmitting substratewas the same as that in the optical layered body of Example 8 but theangle had a negative sign, and for a liquid crystal monitor B′ whereinthe value of angle between the S-polarized light and the slow axis ofthe light-transmitting substrate was the same as that in the opticallayered body of Comparative Example 8 but the angle had a negative sign,the reflectance and the bright-field contrast were evaluated. Theseevaluations showed the same results as those of the liquid crystalmonitor A using the optical layered body of Example 8 and the liquidcrystal monitor B using the optical layered body of Comparative Example8.

Example 9, Comparative Example 9

The optical layered body obtained in Example 1 was disposed such thatthe angle between the S-polarized light and the fast axis of thelight-transmitting substrate was 30°, and the reflectance of the opticallayered body of Example 9 was measured to be 4.56%. On the other hand,the optical layered body was disposed such that the angle between theS-polarized light and the slow axis of the light-transmitting substratewas 30°, and the reflectance of the optical layered body of ComparativeExample 9 was measured to be 4.64%. Therefore, the optical layered bodyof Example 9 was better in anti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example 9had a better bright-field contrast of the display screen, which wasevaluated in the same manner as in Example 1, than the liquid crystalmonitor B using the optical layered body of Comparative Example 9. Theliquid crystal monitor A using the optical layered body of Example 9showed no rainbow interference pattern and the visibility was very wellimproved. In contrast, the liquid crystal monitor B using the opticallayered body of Comparative Example 9 showed no rainbow interferencepattern but was poorer in bright-field contrast and antireflectionproperties than the liquid crystal monitor A using the optical layeredbody of Example 9.

For a liquid crystal monitor A′ wherein the value of angle between theS-polarized light and the fast axis of the light-transmitting substratewas the same as that in the optical layered body of Example 9 but theangle had a negative sign, and for a liquid crystal monitor B′ whereinthe value of angle between the S-polarized light and the slow axis ofthe light-transmitting substrate was the same as that in the opticallayered body of Comparative Example 9 but the angle had a negative sign,the reflectance and the bright-field contrast were evaluated. Theseevaluations showed the same results as those of the liquid crystalmonitor A using the optical layered body of Example 9 and the liquidcrystal monitor B using the optical layered body of Comparative Example9.

Example 10, Comparative Example 10

A polyethylene naphthalate material was molten at 290° C. and was passedthrough a film-forming die to be extruded into a sheet shape. The sheetwas attached onto a water-cooled rapid-cool rotation drum and cooleddown, thereby producing an unstretched film. This unstretched film waspre-heated for one minute at 120° C. and stretched at a stretching ratioof 4.5 times and 120° C. in one direction using a biaxial stretchingtester (Toyo Seiki Seisaku-sho, Ltd.). On one side of the film wasuniformly applied a resin composition for a primer layer that containsan aqueous dispersion of polyester resin (28.0 parts by mass) and water(72.0 parts by mass) using a roll coater. Then, this coated film wasdried at 95° C. and stretched at a stretching ratio of 1.5 times in thedirection orthogonal to the former stretching direction. This yielded alight-transmitting substrate comprising a film (nx=1.81, ny=1.60,(nx−ny)=0.21, thickness=40 μm, retardation=8400 nm) and a primer layer(refractive index (np)=1.56, thickness=100 nm).

Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. The optical functional layer was formed on the primer layer.The obtained optical layered body was disposed such that the anglebetween the S-polarized light and the fast axis of thelight-transmitting substrate was 0°, and the reflectance of the opticallayered body of Example 10 was measured to be 4.37%. On the other hand,the optical layered body was disposed such that the angle between theS-polarized light and the slow axis of the light-transmitting substratewas measured to be 0°, and the reflectance of the optical layered bodyof Comparative Example 10 was 4.79%. Therefore, the optical layered bodyof Example 10 was better in anti-reflection properties.

The liquid crystal monitor A using the optical layered body of Example10 had a particularly better bright-field contrast of the displayscreen, which was evaluated in the same manner as in Example 1, than theliquid crystal monitor B using the optical layered body of ComparativeExample 10. The liquid crystal monitor A′ using the optical layered bodyof Example 10 showed no rainbow interference pattern and the visibilitywas very well improved. In contrast, the display screen of the liquidcrystal monitor B using the optical layered body of Comparative Example10 showed no rainbow interference pattern but was poorer in bright-fieldcontrast than the liquid crystal monitor A using the optical layeredbody of Example 10.

Comparative Example 11

The optical layered body produced in Example 1 was disposed such thatthe angle between the S-polarized light and the fast axis of thelight-transmitting substrate was 45°, and the reflectance was measuredto be 4.59%. The optical layered body was also disposed such that theangle between the S-polarized light and the slow axis of thelight-transmitting substrate was 45°, and the reflectance was measuredto be 4.59%. That is, no difference in reflectance was observedtherebetween, and thus no anti-reflection properties were achieved.

A liquid crystal monitor provided with the optical layered body ofExample 9 was defined as a liquid crystal monitor A, whereas a liquidcrystal monitor provided with the optical layered body of ComparativeExample 11 with an angle between the S-polarized light and the fast axisof the light-transmitting substrate of 45° was defined as a liquidcrystal monitor B. Then, the bright-field contrast was evaluated as inExample 1. As a result, the liquid crystal monitor A using the opticallayered body of Example 9 had a better bright-field contrast of thedisplay screen thereof than the liquid crystal monitor B using theoptical layered body of Comparative Example 11. A liquid crystal monitorprovided with the optical layered body of Comparative Example 11 with anangle between the S-polarized light and the slow axis of thelight-transmitting substrate of 45° was defined as a liquid crystalmonitor B′. The bright-field contrast of the liquid crystal monitor B′was also evaluated in the same manner, and this evaluation showed thesame result as for the liquid crystal monitor B.

Then, liquid crystal monitors using the optical layered bodies ofExamples 1, 7, and 8 were defined as liquid crystal monitors A, whereasa liquid crystal monitor using the optical layered body of Example 9 wasdefined as a liquid crystal monitor B. The bright-field contrast of eachmonitor was also evaluated in the same manner. This evaluation showedthat each of the liquid crystal monitors A using the optical layeredbodies of Example 1, Example 7, and Example 8 had a better bright-fieldcontrast of the display screen thereof than the liquid crystal monitor Busing the optical layered body of Example 9.

Comparative Example 12

The optical layered body produced in Example 3 was disposed such thatthe angle between the S-polarized light and the fast axis of thelight-transmitting substrate was 45°, and the reflectance was measuredto be 4.42%. The optical layered body was also disposed such that theangle between the S-polarized light and the slow axis of thelight-transmitting substrate was 45°, and the reflectance was measuredto be 4.42%. That is, no difference in reflectance was observedtherebetween, and thus no anti-reflection properties were achieved. Aliquid crystal monitor provided with an optical layered body with anangle between the S-polarized light and the fast axis of thelight-transmitting substrate of 45° was defined as a liquid crystalmonitor A, whereas a liquid crystal monitor provided with an opticallayered body with an angle between the S-polarized light and the slowaxis of the light-transmitting substrate of 45° was defined as a liquidcrystal monitor B. The presence of a rainbow interference pattern andthe bright-field contrast were evaluated in the same manner as inExample 1. These evaluations showed that no rainbow interference patternwas observed with either angle and the bright-field contrastscorresponding to the respective angles showed no difference. Incomparison with the liquid crystal monitor A using the optical layeredbody of Example 3, the liquid crystal monitor using the optical layeredbody of Comparative Example 12 showed a poorer bright-field contrastregardless of the angle.

Reference Example 1

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C. and stretched at astretching ratio of 4.5 times and 120° C. in one direction using abiaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). On one side ofthe film was uniformly applied a resin composition for a primer layerthat contains an aqueous dispersion of polyester resin (28.0 parts bymass) and water (72.0 parts by mass) using a roll coater. Then, thiscoated film was dried at 95° C. and stretched at a stretching ratio of1.5 times in the direction orthogonal to the former stretchingdirection. This yielded a light-transmitting substrate comprising a film(nx=1.70, ny=1.60, (nx−ny)=0.10, thickness=28 μm, retardation=2800 nm)and a primer layer (refractive index (np)=1.56, thickness=100 nm).Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. The obtained optical layered body was disposed such that theangle between the S-polarized light and the fast axis of thelight-transmitting substrate was 30°, and the reflectance in this casewas measured to be 4.39%. On the other hand, the optical layered bodywas disposed such that the angle between the S-polarized light and theslow axis was 30°, and the reflectance in this case was 4.45%.Therefore, the reflectances showed difference and the anti-reflectionproperties were achieved. For the bright-field contrast, the presence ofa rainbow interference pattern and the bright-field contrast wereevaluated in the same manner as in Example 1 using a liquid crystalmonitor A where an optical layered body was disposed with an anglebetween the S-polarized light and the fast axis of thelight-transmitting substrate of 30° and a liquid crystal monitor B wherean optical layered body with an angle between the S-polarized light andthe slow axis of 30°. These evaluations showed that the liquid crystalmonitor A had a better bright-field contrast, but it had a retardationof less than 3000 nm. Thus, a rainbow interference pattern was clearlyobserved through polarized sunglasses.

A liquid crystal monitor wherein the value of angle between theS-polarized light and the fast axis of the light-transmitting substratewas the same as that in the optical layered body of the liquid crystalmonitor A of Reference Example 1 but the angle had a negative sign wasdefined as a liquid crystal monitor A′, whereas a liquid crystal monitorwherein the value of angle between the S-polarized light and the slowaxis of the light-transmitting substrate was the same as that in theoptical layered body of the liquid crystal monitor B of ReferenceExample 1 but the angle had a negative sign was defined as a liquidcrystal monitor B′. The reflectances and the bright-field contrasts ofthese monitors were evaluated. These evaluations showed the same resultsas those of the liquid crystal monitor A and the liquid crystal monitorB of Reference Example 1.

Reference Example 2

A polyethylene terephthalate material was molten at 290° C. and waspassed through a film-forming die to be extruded into a sheet shape. Thesheet was attached onto a water-cooled rapid-cool rotation drum andcooled down, thereby producing an unstretched film. This unstretchedfilm was pre-heated for one minute at 120° C. and stretched at astretching ratio of 3.8 times and 120° C. in one direction using abiaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). On one side ofthe film was uniformly applied a resin composition for a primer layerthat contains an aqueous dispersion of polyester resin (28.0 parts bymass) and water (72.0 parts by mass) using a roll coater. Then, thiscoated film was dried at 95° C. and stretched at a stretching ratio of1.8 times in the direction orthogonal to the former stretchingdirection. This yielded a light-transmitting substrate comprising a film(nx=1.66, ny=1.63, (nx ny)=0.03, thickness=100 μm, retardation=3500 nm)and a primer layer (refractive index (np)=1.56, thickness=100 nm).Except for the obtained light-transmitting substrate was used, anoptical layered body comprising an optical functional layer with arefractive index (nf) of 1.53 was obtained in the same manner as inExample 1. The obtained optical layered body was disposed such that theS-polarized light was in parallel with the fast axis of thelight-transmitting substrate (the angle between the S-polarized lightand the fast axis of the light-transmitting substrate was 0°), and thereflectance in this case was measured to be 4.41%. On the other hand,the obtained optical layered body was disposed such that the S-polarizedlight was in parallel with the slow axis of the light-transmittingsubstrate (the angle between the S-polarized light and the slow axis ofthe light-transmitting substrate was 0°), and the reflectance in thiscase was measured to be 4.43%. Therefore, the reflectances showed slightdifference and the antireflection properties were achieved. Further, aliquid crystal monitor provided with an optical layered body with anangle between the S-polarized light and the fast axis of thelight-transmitting substrate of 0° was defined as a liquid crystalmonitor A, whereas a liquid crystal monitor provided with an opticallayered body with an angle between the S-polarized light and the slowaxis of 0° was defined as a liquid crystal monitor B. For each of thesemonitors, the presence of a rainbow interference pattern and thebright-field contrast were evaluated in the same manner as in Example 1.The evaluation showed that no rainbow interference pattern was observedwith either angle, and the bright-field contrasts corresponding to therespective angles showed no difference and were as low as (nx−ny)=0.03.Therefore, these monitors were poorer in the bright-field contrast thanthe liquid crystal monitors using the optical layered bodies of Examples4 and 5.

Reference Example 3

A triacetyl cellulose substrate (TD80ULM, FUJIFILM Corp., nx=1.48026,ny=1.48019, (nx−ny)=0.0007, thickness=80 μm, in-plane phase difference:5.6 nm) was prepared. On this substrate was disposed an opticalfunctional layer (refractive index (nf)=1.53) in the same manner as inExample 1, thereby providing an optical layered body.

The obtained optical layered body was disposed such that the S-polarizedlight was in parallel with the fast axis of the light-transmittingsubstrate (the angle between the S-polarized light and the fast axis ofthe light-transmitting substrate was 0°), and the reflectance wasmeasured to be 4.39%. The obtained optical layered body was disposedsuch that the S-polarized light was in parallel with the slow axis ofthe light-transmitting substrate (the angle between the S-polarizedlight and the slow axis of the light-transmitting substrate was 0°), andthe reflectance was also measured to be 4.39%. Thus, the reflectancesshowed no difference. Although the reflectances showed no difference,the light-transmitting substrate was a triacetyl cellulose substrate andthus this optical layered body had no disadvantage in reflectance. Aliquid crystal monitor provided with an optical layered body with anangle between the S-polarized light and the fast axis of thelight-transmitting substrate of 0° was defined as a liquid crystalmonitor A, whereas a liquid crystal monitor provided with an opticallayered body with an angle between the S-polarized light and the slowaxis of the light-transmitting substrate of 0° was defined as a liquidcrystal monitor B. For each of these monitors, the presence of a rainbowinterference pattern and the bright-field contrast were evaluated in thesame manner as in Example 1. The evaluation showed that no rainbowinterference pattern was observed and the bright-field contrasts showedno difference with either angle. Reference Example 3 confirms that alight-transmitting substrate having no in-plane birefringence which hasbeen conventionally used for liquid crystal display devices has nodisadvantages in the bright-field contrast and does not suffer a rainbowinterference pattern, thereby having no disadvantage in visibility. Theexamples each achieved the visibility as good as that in ReferenceExample 3.

INDUSTRIAL APPLICABILITY

The optical layered body and polarizer of the present invention can besuitably applied to, for example, cathode ray tube display devices(CRTs), liquid crystal displays (LCDs), plasma display panels (PDPs),electroluminescence displays (ELDs), field emission displays (FEDs),touchscreens, electronic paper, and tablet PCs.

The invention claimed is:
 1. A method for improving visibility of an image display device, the image display device comprising: a polarizing element; an optical layered body, which comprises a light-transmitting substrate and an optical functional layer disposed on one surface of the light-transmitting substrate; and a display screen, wherein the light-transmitting substrate has in-plane birefringence, the light-transmitting substrate has a slow axis that extends along a direction showing a greater refractive index and a fast axis that is orthogonal to the slow axis, the light-transmitting substrate has a retardation Re represented by a formula: Re=(nx−ny)×d, in a range from 8,000 nm to 25,000 nm, where nx represents a refractive index in the slow axis direction of the light-transmitting substrate, ny represents a refractive index in the fast axis direction of the light-transmitting substrate, and d represents a thickness of the light-transmitting substrate, the polarizing element has an absorption axis that extends in a horizontal direction relative to the display screen, the optical layered body is disposed at a surface of the image display device, and the image display device is designed to be placed in a room in which the device receives S-polarized light that is in parallel with the fast axis of the light-transmitting substrate, wherein the S-polarized light is light reflected on a floor surface, and the S-polarized light is in parallel with the floor surface, and the method comprising: disposing the optical layered body such that an angle between the slow axis of the light-transmitting substrate and a vertical direction of the display screen is 0°±30°.
 2. The method according to claim 1, wherein the optical layered body further comprises a primer layer disposed between the light-transmitting substrate and the optical functional layer, wherein a thickness of the primer layer is in a range from 3 to 30 nm, provided that: the primer layer has a refractive index np that is greater than the refractive index nx in the slow axis direction of the light-transmitting substrate and that is greater than a refractive index nf of the optical functional layer, as represented by a formula: np>nx and np>nf, or the primer layer has a refractive index np that is smaller than the refractive index ny in the fast axis direction of the light-transmitting substrate and that is smaller than the refractive index nf of the optical functional layer, as represented by a formula: np<ny and np<nf.
 3. The method according to claim 1, wherein the optical layered body further comprises a primer layer disposed between the light-transmitting substrate and the optical functional layer, wherein a thickness of the primer layer is in a range from 65 to 125 nm, provided that: the primer layer has a refractive index np that is greater than the refractive index nx in the slow axis direction of the light-transmitting substrate but that is smaller than a refractive index nf of the optical functional layer, as represented by a formula: nx<np<nf, or the primer layer has a refractive index np that is smaller than the refractive index ny in the fast axis direction of the light-transmitting substrate but that is greater than the refractive index of the optical functional layer, as represented by a formula: nf<np<ny.
 4. The method according to claim 1, wherein the optical layered body further comprises a primer layer disposed between the light-transmitting substrate and the optical functional layer, wherein the primer layer has a refractive index np that falls between the refractive index ny in the fast axis direction of the light-transmitting substrate and the refractive index nx in the slow axis direction of the light-transmitting substrate, as represented by a formula: ny<np<nx. 